* Add user agent and heartbeat headers to Firebase Auth.
* Add app ID header.
* Fix unit tests.
* Remove user agent and heartbeat headers, as backend support is not yet ready.
* Fix typo.
* Update changelog.
* Fix bug where stored ID token is not updated after linking account with email link.
* Update changelog.
* Fix the Game Center and phone auth flows as well.
* Update upcoming release version number in changelog.
* Add debug logging around token expiration.
* Use FIRLogger instead of NSLog.
* Move log statement.
* Add another log statement when access token is updated.
* Remove deprecated APIs from public header.
* Remove deprecated API implementations.
* Also remove deprecated APIs from FIRUser.
* Update unit tests.
* Update sample app.
* Update changelog.