Fixes #35: Add <firebase-auth> support for custom token auth

[sgammon]

This changeset adds a simple function to support signInWithCustomToken in the <firebase-auth> component.

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[sgammon]

I signed it!

[sgammon]

It is a corp CLA under the name Momentum Ideas Co.

[sgammon]

woop,
there it is

[safizn]

Thanks man ! This is a must feature. Wondering why it wasn't included in the release.

[unusec]

Just fund this after I spent some time to create this on my own, this should be merged asap.

[mbleigh]

Great! Thanks for the PR.

[safizn]

@mbleigh @sgammon Thanks for merging it. I cannot figure out how to use it.

I'm getting the following error:
POST https://www.googleapis.com/identitytoolkit/v3/relyingparty/verifyCustomToken?key=KEY 400 ()
page.php:903 my custom error message.
firebase.js:75 Uncaught Error: The custom token format is incorrect. Please check the documentation.

Please provide a simple example. This is what I'm currently using:
<firebase-app name="app" api-key="APIKEY" auth-domain="XXX.firebaseapp.com" database-url="https://XXX.firebaseio.com" ></firebase-app>
<firebase-auth id="auth" app-name="app" provider="custom" on-error="_handleAuthError" ></firebase-auth>
<firebase-query id="query" app-name="app" path="/path/subpath" data="{{data}}" ></firebase-query>
...
ready: function() { this.$.auth.signInWithCustomToken('KEY'); },

[sgammon]

well you will probably need a better key than 'KEY'... lol

but I digress, Firebase's custom token format has changed. so if you are generating tokens in the old 2.x format (we were), you will have to switch over to their new token generator library.

see here for code samples in Java and Node.JS:
https://firebase.google.com/docs/auth/server#create_a_custom_token

basically they changed the token format so that all the developer-provided claims are listed in a claims hash. the JWT is also now signed with your service account's private key, instead of the simple Firebase secret like before.

[safizn]

Oh didn't know that. I was actually using the old tokens. Generating them with the old PHP SDK for the server.
That would be the problem then. Thanks a lot !