Added ability to load tests with requirejs

This allows you to load client side scripts writing against an AMD module definition in node such as requirejs. You can see an example of such from my repo at: https://github.com/mtscout6/javascript-testing-example. You simply run the command `mocha --requirejs` and everything should work the same. I don't know how well this will work with the `--watch` option as it isn't working on windows, so I couldn't test it.