added grunt watch task to continuously run all tests in ff (only)

Grunt is now configured to use the [grunt-contrib-watch](https://github.com/gruntjs/grunt-contrib-watch) plugin to run all tests as you make changes to the source code or test suites in FireFox (only) by typing the following at the command line:

```bash
grunt watch
```

Make sure to run `npm install` first to get the latest dependencies.

This resolves #135, but there are improvements that can be made in this area. Currently, tests are running in FF w/ Selenium, which is obviously faster than in all 3 browsers, but I'd like to look into using Phantom JS to see if it's any faster. Going to open a separate issue for that. Also, as test suites grow, we may want to limit which ones are run (i.e. only unit tests).