add JSHint grunt task and use it on /src/**/*.js + fix existing tests

ThoughtWorker in Melbourne just doing a couple small things with jetpets. I noticed your PR on the main jetpets repo has been outstanding for a little while now and I'm inclined to work off the newer code base, so I forked your fork. Hopefully we can get it all merged back down at some point, but I figured I'd pass along these couple of small tidying commits in the meanwhile.