Add watch-tests and server-test-watcher tasks to Gruntfile to enable liv...

...e reloading of test pages when developing.

I'm not sure about the naming of the tasks, and am happy to change them. I also added both a concurrent task which runs both the server and the watcher and a specific watch task. Unfortunately, the watch task cannot be named 'watch' due to this bug https://github.com/gruntjs/grunt/issues/1260.
Grouping them in such a fashion won't work: both `connect:keepalive` and `watch:tests` will never end, so `connect:keepalive` is the only task that will run in your example. `grunt-concurrent` solves problem by running them simultaneously.

Would you like me to rename the task to `livereload-tests`?