Added Feedly provider

This patch adds OAuth2 authentication support for Feedly Cloud API to django-allauth. The Feedly API is currently in private beta, developers need to apply for a key/ secret pair for the production API.
Nevertheless, given the key/ secret, authentication is pretty standard OAuth2 and works.

I ran the tests successfully on Python 2.7.5. I used __future__.unicode_literals to support unicode in the profile JSON. I hope this works across all supported Python versions.