Fix #577 - Enable subtitles explicitely
Fix #717 - Insert subtitle track in ready event
Force native subtitles for certain devices

Namely iPhone and others that play video in the fullscreen media player

Also forces native subtitles when using a touch device and
native_fullscreen is configured

Refs #717
Merge pull request #718 from flowplayer/bug/577

Fix #577 - Enable subtitles explicitely