* Replacing FIR_SWIFT_NAME macro with NS_SWIFT_NAME.
This pushes the minimum Xcode version to 7.3, as NS_SWIFT_NAME was
limited before that version (which is why the macro was introduced in
the first place).
* Fixed FIRMessaging header
* Move Auth notification constant into Core.
SDKs that want to listen for the internal Auth notifications needed
to copy the notification strings to their own SDK instead of relying
on Auth's definition in order to avoid a dependency on Auth. By moving
them to Core, SDKs can use the constants without taking on another
dependency.
* Remove Auth stub, add app instance to notification.
Sebastian Schmidt