Provide shortcut for configuring DEFAULT_FIREBASE provider

[jeffbcross]

providers: [defaultFirebase('https://myfirebase')]

instead of

providers: [provide(DEFAULT_FIREBASE, {useValue: 'https://myfirebase'})]

[jeffbcross]

FYI I renamed DEFAULT_FIREBASE to FirebaseUrl, and DEFAULT_FIREBASE_REF to FirebaseRef. So the function name defaultFirebase could be improved.

[jeffbcross]

Closed by dddd0b3