Added Facebook Login for Android

[t-moe]

This should fix #45.

It works in my test-enviroment, but I haven't tested it with the demo app or after merge to 3.4.2.
So please test it first before merging :). Also the docs regarding fb@android might need some polish.

[EddyVerbruggen]

Sweet! I will test this asap.

[t-moe]

Ok I've added some more bugfixes which were related.

One thing still bothers me, although it's not facebook-login related:
If you add a AuthStateListener it can be called multiple times per login. I'm not 100% sure but I think it's not related to this project, but rather to the firebase api on android. See firebase/quickstart-android#80

Have you experienced this as well?

[EddyVerbruggen]

Yeah I think I have.

I tested your changes btw and it all seems to work very well, so thanks a LOT!

I will polish a few minor things after merging it.

[EddyVerbruggen]

I've also invited you to collaborate on this repo. PR's are still the way to go but you'll be able to merge them yourself, or in case of a trivial fix (code or doc) you can just push it if you like.

[t-moe]

Thanks and thanks for the invitation.
I've accepted it, but I'm not sure how much I'm going to contribute.
I'm still just playing around with NativeScript and I'm not yet sure whether to use NativeScript or Ionic for my App. I'm not happy with the stability of the NativeScript Build Process and also it's way too slow (even with livesync).