Have npm install phantomjs locally as a dependency.

I noticed that you are requiring the user to install phantom independently, which is just an additional obstacle and makes deployment to servers more difficult, but I knew that the test runner I use automatically installs phantom through npm, so I brought that in here. Now a simple "npm install" should be all that's necessary to get up and running.

Originally I was motivated by wanting to contribute to [makemeasandwhich](https://npmjs.org/package/makemeasandwich) but this will have to do :)

Nice library btw. Super simple to use.