Enables remote execution for browserstack

If the flag ``--browserstack-local`` is set to **false**, massah will not launch a local browserstack server, and then use http://hub.browserstack.com/wd/hub directly. This is useful for testing public URLs. 
This flag is set to **true** by default.
My guess is that we should rename **local-wait** to something else, thus we comply with browserstack's terminology: http://www.browserstack.com/automate/capabilities.