Feature: Fixture Files

The first commit is a bugfix.

The second unifies the two drivers to use the same `run` method, and simply offer up different command lines to produce the output. This was done so the rhino + envjs strategy could use a test html page just like the phantomjs driver does.

The third commit enables fixture files to be specified on the command line or in the rake task, which are included wholesale as html into the test wrapper page. This is useful for tests that depend on some markup to be present before running.