grunt-phantomcss refactor

I've refactored the task to perform the way I would expect it to.

* Task handles installation of its dependencies (PhantomJS/PhantomCSS)
* Tests and CasperJS/PhantomCSS setup are now completely separate
  * CasperJS/PhantomCSS setup is handled entirely by the task
  * Users simply create PhantomCSS tests exactly like the [example from PhantomCSS](https://github.com/Huddle/PhantomCSS#example)
* Multiple test files are supported
* Source screenshots are treated as fixtures (made to be committed to the repo)
* Diff/failure screenshots are treated as test results (should be ignored, cleaned before runs)
  * Diff/failure screenshots are stored in the results directory
* Viewport size is configurable via options
* CasperJS log level is configurable via options
* Failed/successful tests are reported in task output
* Task fails when tests fail
* Task tests a zero-dependency test page with baseline screenshots
* When testing local files, they're accessed directly instead of served
  * Task is no longer responsible for launching a webserver to serve local files, this should be done with something like [grunt-express-server](https://github.com/ericclemmons/grunt-express-server) if necessary
* Patterns and coding style have been adjusted to more closely match [grunt-contrib tasks](https://github.com/gruntjs)
* Finally, documentation has been updated to include complete examples of Grunt configuration and tests

Fixes #1.
Resolves #2.
Fixes #3.
Resolves #4.
Resolves #5.