Support Other Than Amazon Hosts ?

Just tried to use digital ocean's spaces but it failed:

```
s4cmd ls
[Runtime Exception] An error occurred (InvalidAccessKeyId) when calling the ListBuckets operation: The AWS Access Key Id you provided does not exist in our records.
Traceback (most recent call last):
File "/usr/local/bin/s4cmd.py", line 1928, in main
CommandHandler(opt).run(args)
File "/usr/local/bin/s4cmd.py", line 1557, in run
CommandHandler.__dict__[cmd + '_handler'](self, args)
File "/usr/local/bin/s4cmd.py", line 129, in wrapper
ret = func(*args, **kargs)
File "/usr/local/bin/s4cmd.py", line 1628, in ls_handler
self.pretty_print(self.s3handler().list_buckets())
File "/usr/local/bin/s4cmd.py", line 129, in wrapper
ret = func(*args, **kargs)
File "/usr/local/bin/s4cmd.py", line 694, in list_buckets
for bucket in self.s3.list_buckets().get('Buckets') or []:
File "/usr/local/bin/s4cmd.py", line 402, in wrapped_method
ret = getattr(self.client, method)(*args, **merged_kargs)
File "/usr/local/lib/python3.7/dist-packages/botocore/client.py", line 357, in _api_call
return self._make_api_call(operation_name, kwargs)
File "/usr/local/lib/python3.7/dist-packages/botocore/client.py", line 661, in _make_api_call
raise error_class(parsed_response, operation_name)
botocore.exceptions.ClientError: An error occurred (InvalidAccessKeyId) when calling the ListBuckets operation: The AWS Access Key Id you provided does not exist in our records.
```

Are other hosts supported ?

Start in s4cmd.py at the S3 client setup and the ls_handler/list_buckets path shown in the traceback. Determine how a non-Amazon endpoint such as DigitalOcean Spaces should be configured, then verify that authentication and bucket listing work against that host.

Written by the indexing model from the issue text.