Fix video controls on iPad.

This PR provides fixes for [BLD-541](https://edx-wiki.atlassian.net/browse/BLD-541), [BLD-598](https://edx-wiki.atlassian.net/browse/BLD-598).

In this PR were done following things:
- Disable edX controls for iPhone (native controls are used).
- Disable volume and speed controls for iPad (see [iOS-Specific Considerations](https://developer.apple.com/library/safari/documentation/AudioVideo/Conceptual/Using_HTML5_Audio_Video/Device-SpecificConsiderations/Device-SpecificConsiderations.html#//apple_ref/doc/uid/TP40009523-CH5-SW1)).
- edX controls becomes visible after click on video or play placeholder to avoid issues with YouTube API (see issues [#5204](https://code.google.com/p/gdata-issues/issues/detail?id=5204) and [#5415](https://code.google.com/p/gdata-issues/issues/detail?id=5415)).
- Captions becomes visible just after full initialization of video player. 
- Fix blinking of captions after video player initialization.

Screenshots are available in the [BLD-541](https://edx-wiki.atlassian.net/browse/BLD-541).

@valera-rozuvan , @jmclaus please review.