Automatically unlock/enable audio context on iOS

On iOS audio is effectively muted until user interaction, as described here: http://paulbakaus.com/tutorials/html5/web-audio-on-ios/

This code automatically tries to enable it when `new Howl()` is executed and tries again on the first touch—unless it's already marked as enabled. This auto-enable function can be disabled by setting:

    Howler.automaticallyEnableiOSAudio = false;

Tested on iOS 7. Fixes #22, #87

This iOS limitation+workaround should probably be documented too.