Adds support for drag&drop external subs directly into the player.

This is something I've just finished at the same time Popcorn Time was going offlane.

Allows for adding custom subtitles with Drag&Drop.

· loadSubtitle now checks the file extension, returns false if it isn't
'.srt' (needed to allow event propagation if dropped something other
than a sub)

Also 	I cleaned up subtitle loading. It now makes use of request (already required by the app) instead of http, and added some error handling -- All errors fallback to the TextTrack.onError event.