Optionally pass in an existing instance of Casper

I'm not completely sure how many people would find this useful or if my case is peculiarly specific. I've a fork of the project (https://github.com/thingsinjars/PhantomCSS) and the only difference is I'm allowing the instance of Casper used for the tests to be created by another process.

It's basically having `casper = options.casper || casper;` in the init.

https://github.com/thingsinjars/PhantomCSS/blob/master/phantomcss.js

For my use, I'm pulling PhantomCSS in as a subrepo of [GhostStory](https://github.com/thingsinjars/GhostStory) which does cucumber-style CSS tests. 

PhantomCSS is providing the 'Then the "Element descriptor" should look the same as before' step.