VPAID integration

Hello,

This PR adds basic support for linear VPAID ads. PR is based on squashed branch. I've kept [original branch](https://github.com/mente/videojs-vast-plugin/tree/vpaid) with all commits in if you like.

Generally what has been done:
* use forked version of vast-client to correctly detect VPAID (relies on dailymotion/vast-client-js/#52)
* if VPAID is found in VAST response - use it
* small ad time indication (for VPAID only, can be applied to VAST playback as well)
* localization support for text used in plugin (closes #44). Requires video.js version 4.7.3
* tests (though I don't like it's quality)

Tested on Chrome stable latest, IE9, Firefox stable latest, iOS7.1, iOS8.0, Android 4.3 (Chrome).
PR is pretty big, looking forward to comments and ideas for improvement.

Regards,
Alex