Refactor videoplugins.js

Refactor videoplugins.js (vjs.TextTrack.prototype.load)

It now makes use of request (already required) instead of http to
download subs. (Cleaner code)
Added some 'error handling'. Errors fallback to console.error. This will
need to be replaced by the prefered error reporting method of the app
(when we have one) Should be a safer and cleaner to read.
I could've throw them instead, and let process.on('uncaughtException')
catch them.. but it didn't seem right.