Bug 955901 - Video Player doesn't shows subtitles

The files I modified in this commit:
   gaia/apps/video/manifest.webapp
   gaia/apps/video/js/metadata.js
   gaia/apps/video/js/db.js
   gaia/apps/video/js/video.js

I created  gaia/apps/video/js/subtitle.js to implement subtitle display feature.