Add functionality to watch files and directories.

Aloha! I thought it would be nice to have the ability to watch certain files/dirs for changes and run unit tests (with 'npm test') when a change is detected. Now that I think of it, I guess it is more of a utility and maybe shouldn't be defined as a task, since its persistance forces it to be used through a separate target, by itself. Interested to hear your thoughts. Thanks!

-Ben