Webpack-based compilation

This removes browserify as our client compilation system and adds a few nice features:

* Faster compilation time
* Common module splitting
* AMD support (canjs is now loaded straight from AMD)
* External source maps

It also opens up the door to other neat features:

* es6-style modules (once webpack's author adds support -- soon)
* hot code reloading (omg omg?)
* Larger-scale code splitting once the app grows
* Separate static .css file instead of JS adding styles
  (the .css file would be built based on require() calls in the
   same compilation step)