Added Tern + more js2-mode integration.

Not sure if you'd want the tern bindings under `SPC` or not, but I've left them as they are for now. This basically uses js2 for all `.js` files (as I've found this to be superior to the minor mode) and then attach tern completion to that.

To get tern to work the user just needs to install the tern NPM module, so basically follow the standard tern instructions on the website. It's very well documented.