Add selenium with chrome to the test suite

@jnicklas - This PR adds selenium with chrome to the tests - There is one test failure currently   

Failure/Error: expect do
 expected TestApp::TestAppError but nothing was raised
 # ./lib/capybara/spec/session/reset_session_spec.rb:36:in `block (2 levels) in <top (required)>'

This doesnt occur if just the chrome specs are run, or just the firefox, but if both are run the error occurs.  I'm 99% sure this is because of the way Capybara::Server caches and looks up the port for server instances by object_id, meaning that even though I create a separate test session for chrome which creates a server object, that server object doesnt actually get booted (doesnt error either), and instead the tests are hitting the server object created for the firefox session.  This doesnt make any difference for most of the tests except the one failing since it checks the internal error state of the server object it thinks is handling requests, but ends up not. Not sure what the best way to fix that error would be, any suggestions?