MDL-44342 airnotifier: Initial work, first version of the plugin
MDL-44342 airnotifier: Added support for the new workflow of registering devices

And also for request access keys to Airnotifier
MDL-44342 airnotifier: rest.php should return success information

Javascript should handle properly errors, also, the eye and spinner
should be synchronized with the remote operation
MDL-44342 airnotifier: Add support for Android notifications
MDL-44342 airnotifier: Use system context instead user.

Added missing capabilities and language strings
MDL-44342 airnotifier: Unit tests for external functions (web services)
MDL-44342 airnotifier: Add information about the message in the payload.

In order to check if is a notification or just a private message
MDL-44342 airnotifier: Include missing defined and required lib in externallib.php
MDL-44342 airnotifier: Add support for shifter in the YUI Module

This commit includes also some integrator review related fixes