Automatically unlock/enable audio context on iOS

On iOS audio is effectively muted until user activation, as described here: http://paulbakaus.com/tutorials/html5/web-audio-on-ios/

This code automatically tries to unlock it when `new Howl()` is executed and tries again on the first touch. This can be disabled by setting:

    Howler.automaticallyEnableiOSAudio = false;
Properly wait for the next touch. Make the method public
Expose supported `codecs` with a new global method.
Correctly clean up end timers
Merge branch 'pr/168'
Merge branch 'pr/163'

Conflicts:
	howler.js
Code cleanup
Create documentation for iOS update
v1.1.22