Adds initial Evernote OAuth support

I ended up copying/pasting the entire `OAuthCallbackView` to fix a `KeyError` because Evernote doesn't return an `oauth_token_secret`:

https://github.com/pztrick/django-allauth/compare/pennersr:master...feature/evernote_oauth#diff-6be7fa52b030f92e42ec8b081697b3cfR57

We could make the `OAuthCallbackView` use `access_token.get('oauth_token_secret', None)` to avoid the `KeyError` but wasn't sure if this is a desired approach / if the KeyError should in fact occur for other services.