Adding the csslint Grunt task

Introducing CSSLint for cleaner CSS! We JSHint, so why not CSSLint, too?
Merge branch 'pr/109' into develop

Merges changes from #109 into develop, adding the csslint task as an
available option in our Gruntfile.

This does not require csslint on a `grunt prod`, but can be useful for
checking CSS work. In the future we may want a set of rules to go along
with this.

Fixes #109