Contrib JavaScript Layer

JavaScript changes:
* moved JavaScript/js2-mode into a contrib layer, `javascript`
* added js2-refactor
* changed key bindings to be more spacemacsy

I also created a `spacemacs/declare-prefix-for-mode` function, so you can define prefixes that will disappear when you are in a different mode.

Feel free to tell me if my binding choices are bad. I'm coming from an Emacs background and am not yet used to all the vim conventions.