added grunt-contrib-watch with a testunner.html to use livereload

Added `grunt-contrib-watch` and use `livereload` to reload the `testrunner.html` for every file change. This does require the user to install the livereload extension for their browser of choice. I've tested in Chrome so far and it works fine. Also copied the `client.html` file to a `testrunner.html` and adjusted the paths so the user wouldn't be required to add the livereload script in the `node_modules/intern` folder. This gives some nice feedback while developing.