Goog closure integ

I considered opening and issue to discuss this, but as I explored the issues I realized that just making and documenting the changes would be more clear, so here you go. 

This change makes it so that orgs (like ours) that use the official Google Closure and which have our own build process can still use video.js in a structured way without fear of conflicting closure library or compiler versions. The change should be totally backwards compatible and is entirely opt-in.

My humble recommendation is to first look at `docs/google-closure.md` first for the motivation for this change. Then I'd look at either of `Gruntfile.js` or `build/source-loader.js` for the implementation.