Build system improvements

I've added a watch task, a jshint task, and added a script for testing to the package.json.

Watch will allow for easier development by automatically running tests in response to changes in source files. 

The jshint task currently only runs against the gruntfile but it's the beginnings of enforcing some consistent code style/standards. (If we can decide on a jshint configuration for jshint I'll happily start chipping away at that)

The test script is a formality at this point, but if you ever wanted to plug into Travis CI this would serve it's purpose. Travis CI is great as it'll run for each check-in, pull request etc so that you can be sure nothing is broken :)