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;

Tested on iOS 7. Fixes #22, #87