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.
__Massah__ should be able to run without a `.massah.js` therefore this option should have a fallback or be ser as a default value in `helper.js`.