Streamlined css flow

Source maps seem to be mostly working for css. I'm not sure what the exact behavior is expected to be. `main.css` file is no longer created in `app` and is instead created directly in `dist`. I made the switch to `normalize-scss` and am importing that in `_base.scss` so we no longer have to copy over `bower_components`.

Fixes #56.

*This should be merged after #55.*

@pdehaan r?
Do we need sourcemaps here AND in the autoprefixer task?