Adding OAuth2 for Authentication

I wanted to keep the pull req small, so I've only added Google and LinkedIn providers for now. I have been using this in production now for over 6 months and it's working just fine.

I haven't modified the build scripts or nugets since I'm not sure how much control you want over that process, but happy to do it if you would like that as well.

Tests - I couldn't get the web endpoint tests running regardless of running localhost:50000 or 50095 and I *think* I had sqlLite setup ok - so not sure what you need me to do there, but happy to include tests if you need that for this code - if I can get some help running locally

If this pull req gets accepted, then I'm happy to contribute the other providers such as Facebook, Windows Live, Github etc, but I didn't want to overwhelm the pull req.