Detect and use system proxy

Currently, the workflow's python script does not respect the globally set proxy:

```
20:25:11 workflow.py:2133 DEBUG Last run version : 1.2.0
20:25:11 workflow.py:2181 DEBUG update_data : None
20:25:11 workflow.py:2350 DEBUG Got password : com.fniephaus.gmail:gmail_credentials
Traceback (most recent call last):
File "gmail.py", line 271, in
main()
File "gmail.py", line 133, in main
refresh_cache([label])
File "/Users/Sepehr/Dropbox/dev/dotfiles/conf/app/alfred/Alfred.alfredpreferences/workflows/user.workflow.A06A4C55-CF30-4DAA-9ED8-A6C15CA10B7F/gmail_refresh.py", line 122, in refresh_cache
gmail_service = build('gmail', 'v1', http=http)
File "/Users/Sepehr/Dropbox/dev/dotfiles/conf/app/alfred/Alfred.alfredpreferences/workflows/user.workflow.A06A4C55-CF30-4DAA-9ED8-A6C15CA10B7F/oauth2client/util.py", line 132, in positional_wrapper
return wrapped(*args, **kwargs)
File "/Users/Sepehr/Dropbox/dev/dotfiles/conf/app/alfred/Alfred.alfredpreferences/workflows/user.workflow.A06A4C55-CF30-4DAA-9ED8-A6C15CA10B7F/apiclient/discovery.py", line 198, in build
raise HttpError(resp, content, uri=requested_url)
apiclient.errors.HttpError:
```

That's unfortunate to say that Google blocks access to its Gmail API from Iranian IPs. So, basically, everyone needs to use some kinda VPN or proxy to access it here.

That'd be great (and much appreciated) the workflow could detect and use the system's set proxy.