Add capability to start phantomjs with remote debugger in ruby webdriver

I love being able to use phantomjs as a webdriver in ruby, but miss the remote
debugger which was so easily exposed via poltergeist.

This pull adds the capability to start the phantomjs service with the remote
debugger listening.

Still not sure how to expose this in the actual browser or bridge. Might require
adding another driver extension, maybe HasDebugger with #debug. Thoughts?

(I have signed the CLA.)