Add methods to trade auth code and verify tokens

I used the *responses* package to mock requests, which differs of your current test suite...  I find *requests-mock* not so easy to assert on.

This piece of code was originally written for a proof-of-concept, hence fully open to suggestions/feedback/wipeout :)
Yes, good idea! 

Currently ``verify_token`` returns the profile info. If we introduce some scope checking, we should remain consistent, thus either:

* add another method ``fetch_profile(token)`` and change ``verify_token(token, scope)```to just return ``True/False`` ? 
* or add a new kind of exception, raised in ``verify_token`` when scope does not match

What do you think ?