fix(build): don't override generated css files

Fixed #231
CSS was previously generated through compass and moved into a `main.css` file,
but later overwritten by `concat` task generated by `usemin`. This removes the
usemin block around the CSS link and adopts the same schema used in
`generator-webapp`.
fix(build): use Usemin for all CSS.
Merge pull request #248 from passy/css-fix

fix(build): don't override generated css files