fix bug 1076863 - Add CSSLint to dev box

@stephaniehobson Mentioned wanting a CSS validator so I think this is our best bet.  This PR doesn't contain any usage settings yet, but we can figure that out as we go along, like:

-  Should we add it to compile-stylesheets?  (I vote no, but...)
-  Should each of us add it as a pre-commit hook?
-  Should we add individual lint settings?

To check, do `vagrant provision` from the host and then ssh into the box, type "css" and then press tab;  "csslint" should be there.