Update to support Firebase 3.x and Angularfire 2.x

[petrusek]

There is a new version of Firebase and Angularfire, which are not compatible with the old extern files. It would be great to update them, so also the projects depending on these externs could migrate easily.

Note: If it is easier for you to write some notes on how to re-create these externs (what tools to use and how - hope it is not just manual work), I could help.

[petrusek]

Actually I found out, that

1. externs for Firebase 3.x are provided by the Firebase team in their npm package. Unfortunately, they are not in the bower package (yet).
2. externs for Angularfire 2.x are not needed, as the original source (within the bower package) comes with annotations, so it can be compiled together with my app. WRONG

[petrusek]

I was wrong in 2., about AngularFire, it is written in an IIFE and does not export any types/symbols for Closure Compiler, so it can't be compiled together with my app. Or at least I don't know how.

[adelespinasse]

More specifically, externs for the current version of Firebase are here: https://github.com/firebase/firebase-js-sdk/tree/master/packages/firebase/externs