Frontend development via `grunt watch`

In order to support our web client development in js/sass, running
`grunt watch` now watches for sass and compiles it into compiled/css.
It also copies js in the static/ folder into static/compiled/js.
Flipping a switch in config.js (optimized: none to optimized:uglify)
will turn on uglification via requirejs, but will slow down the grunt
task. @jnwng @wesvetter @chushao