Support default env credentials (e.g. needed in Google Colaboratory (Colab)) + workaround for now

Google Colaboratory provides these instructions for using PyDrive: https://colab.research.google.com/notebooks/snippets/drive.ipynb#scrollTo=bRFyEsdfBxJ9

Running the same code, but substituting `pydrive2` for `pydrive`, produces errors as collapsed here.

```
FileNotFoundError Traceback (most recent call last)
/usr/local/lib/python3.7/dist-packages/oauth2client/clientsecrets.py in _loadfile(filename)
120 try:
--> 121 with open(filename, 'r') as fp:
122 obj = json.load(fp)

FileNotFoundError: [Errno 2] No such file or directory: 'client_secrets.json'

During handling of the above exception, another exception occurred:

InvalidClientSecretsError Traceback (most recent call last)
9 frames
/usr/local/lib/python3.7/dist-packages/pydrive2/auth.py in LoadClientConfigFile(self, client_config_file)
476 client_type, client_info = clientsecrets.loadfile(
--> 477 client_config_file
478 )

/usr/local/lib/python3.7/dist-packages/oauth2client/clientsecrets.py in loadfile(filename, cache)
164 if not cache:
--> 165 return _loadfile(filename)
166

/usr/local/lib/python3.7/dist-packages/oauth2client/clientsecrets.py in _loadfile(filename)
124 raise InvalidClientSecretsError('Error opening file', exc.filename,
--> 125 exc.strerror, exc.errno)
126 return _validate_clientsecrets(obj)

InvalidClientSecretsError: ('Error opening file', 'client_secrets.json', 'No such file or directory', 2)

During handling of the above exception, another exception occurred:

InvalidConfigError Traceback (most recent call last)
in ()
16 uploaded = drive.CreateFile({'title': 'Sample file.txt'})
17 uploaded.SetContentString('Sample upload file content')
---> 18 uploaded.Upload()
19 print('Uploaded file with ID {}'.format(uploaded.get('id')))

/usr/local/lib/python3.7/dist-packages/pydrive2/files.py in Upload(self, param)
495 self._FilesPatch(param=param)
496 else:
--> 497 self._FilesInsert(param=param)
498
499 def Trash(self, param=None):

/usr/local/lib/python3.7/dist-packages/pydrive2/auth.py in _decorated(self, *args, **kwargs)
58 self.auth.ServiceAuth()
59 else:
---> 60 self.auth.LocalWebserverAuth()
61
62 # Initialise service if not built yet.

/usr/local/lib/python3.7/dist-packages/pydrive2/auth.py in _decorated(self, *args, **kwargs)
122 self.LoadCredentials()
123 if self.flow is None:
--> 124 self.GetFlow()
125 if self.credentials is None:
126 code = decoratee(self, *args, **kwargs)

/usr/local/lib/python3.7/dist-packages/pydrive2/auth.py in GetFlow(self)
565 config in self.client_config for config in self.CLIENT_CONFIGS_LIST
566 ):
--> 567 self.LoadClientConfig()
568 constructor_kwargs = {
569 "redirect_uri": self.client_config["redirect_uri"],

/usr/local/lib/python3.7/dist-packages/pydrive2/auth.py in LoadClientConfig(self, backend)
454 )
455 if backend == "file":
--> 456 self.LoadClientConfigFile()
457 elif backend == "settings":
458 self.LoadClientConfigSettings()

/usr/local/lib/python3.7/dist-packages/pydrive2/auth.py in LoadClientConfigFile(self, client_config_file)
478 )
479 except clientsecrets.InvalidClientSecretsError as error:
--> 480 raise InvalidConfigError("Invalid client secrets file %s" % error)
481 if client_type not in (
482 clientsecrets.TYPE_WEB,

InvalidConfigError: Invalid client secrets file ('Error opening file', 'client_secrets.json', 'No such file or directory', 2)
```

The google-colab package would [monkey-patch pydrive](https://github.com/googlecolab/colabtools/blob/7c5503e6cdaa23971c08b5a3f5a6d23667a753cb/google/colab/_import_hooks/_pydrive.py) to avoid these errors.

To monkey-patch pydrive2 in the same way, you need to:

```python
import httplib2
from oauth2client.contrib.gce import AppAssertionCredentials
from pydrive2.auth import GoogleAuth

old_local_webserver_auth = GoogleAuth.LocalWebserverAuth

def LocalWebServerAuth(self, *args, **kwargs):
if isinstance(self.credentials, AppAssertionCredentials):
self.credentials.refresh(httplib2.Http())
return
return old_local_webserver_auth(self, *args, **kwargs)

GoogleAuth.LocalWebserverAuth = LocalWebServerAuth
```

Then, you can use the same code as in Google Colaboratory's instructions.