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