Support for PhantomJS

I added support for PhantomJS. It passes all the tests except the "Alert API" test, unfortunately [PhantomJS doesn't support the Alert API](https://github.com/detro/ghostdriver/issues/20). To work around this, I broke the common tests into "common" and "alert", when the test is run PhantomJS doesn't have to pass the "alert" tests.

Presently you can set the PhantomJS executable path by passing an option to the JVM. For instance, in my `~/.lein/profiles.clj` file I have the following:

  {:user {...
          :jvm-opts ["-Dphantomjs.binary.path=/usr/local/bin/phantomjs"]}}

The path can also be set via the `DesiredCapabilities` object, as it is for Chrome.

Everything else works as expected. Thank you for putting this library together, it's been a great help to me already!