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.
Wondering if we should rename the option to **use-proxy** or **use-tunnel** ? Just concerned about the clash with 'local-wait', although I feel we should now rename this too.