Webpack dev server

This branch switches development mode over to the webpack-dev-server, which provides fast rebuilds.
 - Modified default file extension for React files from .js to .js (backwards compatibility issue)
 - Modified serve command to use webpack-dev-server instead of connect
 - Added generation of a CSS file for new components
 - Added basic test for main component
 - Moved webpack configuration out to two separate files (one for webpack-dev-server, one for the distribution build)

Apologies again for the giant commit.