filter connections by both provider and user Id

Currently login_handler assumes that provider_user_id is unique across different providers.
That's not guaranteed to be true, for example paulftw at Github and paulftw at Twitter are different people.
Facebook uses integer numbers to identify users, these may collide with
some other (existing or future) OAuth provider.
Merge pull request #16 from paulftw/patch-1

filter connections by both provider and user Id
Add travis file
Merge branch 'develop' of github.com:mattupstate/flask-social into develop
Install Flask-Testing for travis
Convert tests to use mock data instead of actually hitting Twitter. What a stupid idea that was before!
Move config class
Update README.rst
Update test dependencies
Import within _query function in case someone is not using MongoEngine
Polish
Update docs
Add flask-peewee support
Add more travils build requirements
Add legacy test functions
Update CHANGES and bump required version of Flask-Security
Bump version number to 1.6.0