Added a bower.json file for bower support

It would be nice if we can have [Bower](http://bower.io/) support for Mocha.

Assuming you agree to register bower as a module, users should be able to run `bower install mocha`, and they should then be able to access mocha via `./bower_components/mocha/mocha.js`.

And as an added bonus: it doesn't conflict with `component.json`.

This pull request tackles #584.