Added an option for adding key/values to phjs' page.customHeaders

Yo,

I added an option to add custom headers to phantomjs.

Additionally I added:

* The tests to prove it works (including a small `expressjs` server to bounce headers).
* A listener in the `Gruntfile.js` to print debug info. (Not required, just useful.)
* Some documentation about options for the `README.md`