Register 'messagebird' channel

[WouterFlorijn]

Currently, use of this package is inconsistent. In most cases (for example in the via method), the channel needs to be MessagebirdChannel::class. However, when routing an anonymous notification, the channel needs to be 'messagebird':

Notification::route('messagebird', $phoneNumber)->notify(...);

I think Notification::resolved could be used in the MessagebirdServiceProvider to accomplish this. See for example the Nexmo implementation.

[zenire]

@petericebear What is needed to resolve this issue? Looking if I can help somehow, but I see a PR has been made already.