lit_firebase_auth-v0.3.0

• BREAKING: Updated to Firebase Core 0.5.0 (thanks @ciriousjoker)
• BREAKING: User renamed to LitUser
• BREAKING: FirebaseUser renamed to User
• BREAKING: getSignedInUser() is no longer asynchronous
• docs: various documentation updates
• Additional setup is now required to ensure that Firebase Core is initialized. Please see the documentation or example for details.