Extract basic OAuth2 functionality from SportTracks so it can be used elsewhere

[mduggan]

I started looking at doing #86 Google Fit support, but the Google API libraries don't have code for it beyond doing OAuth2 - and that was already the ugliest part of the way I implemented #93 Google Drive support, since it doesn't support requests.

Instead, I thought it might be nicer to pull the OAuth2 functionality out of SportTracks and make it available for other services.

There are 2 small behaviour changes to the sporttracks code:

1. The token refresh does not pass a redirect_uri - that was hard coded so it can run outside Django, and shouldn't be needed anyway according to http://tools.ietf.org/html/rfc6750#section-3
2. The download now uses a requests.Session so it will recycle connections when downloading multiple activities. I'm not sure if this has potential to trigger some sort of rate limiting?

If you like this refactor I'll probably rewrite #93 to drop the google API client library - it doesn't do much and requests is so much nicer. Will also give #86 a bash.