[2.x] Implement shortcode support

[dwightwatson]

As suggested in #13, this implements shortcode support with Nexmo. Pulled this out of our app to break off into a package but would be better in the official notification channel.

A couple of notes: -

• I called the channel shortcode for brevity, but it isn't quite as "namespaced" as the nexmo channel. I'm not sure there is a precedent for multi-word channel names - would nexmo-shortcode be better?

• At the moment I've implemented it with support for an array which Nexmo\Message\Shortcode will adapt into an instance under the hood. Would you want me to implement a fluent interface to build up a shortcode message as well?

• I added a base TestCase that uses MockeryPHPUnitIntegration - this fixes an issue with PHPUnit complaining that the tests didn't actually perform any assertions (despite the fact they set expectations on mocks).

1) Illuminate\Tests\Notifications\Channels\NexmoShortcodeChannelTest::testShortcodeIsSentViaNexmo
This test did not perform any assertions

Happy to sort out a PR for the docs as well if you decide to bring this in.