Enable yas snippets for JavaScript

I've duplicated the js-mode snippets from https://github.com/overtone/emacs-live/tree/master/packs/live/power-pack/lib/yasnippet/snippets/js-mode to the directory that emacs live sets yas-snippet-dirs to https://github.com/overtone/emacs-live/tree/master/etc/snippets.

Also, given that the mode used by emacs live for JavaScript files is js2-mode I've also added a snippets dir for that mode too.