add basic browserstack support

[yanatan16]

This is basic BrowserStack support. It does --list-available-browsers and runs tests when --browserstack is passed in. It requires browserstack_username and browserstack_key values in .zuulrc.

Known Bugs:

• Sometimes doesn't get Passed responses with 4 browsers running
• Running two or more types of browsers will not queue correctly in BrowserStack: You'll get errors saying you can only have X number of tests running at once instead of tests queuing up to run later.

See #124

[defunctzombie]

wondering if --platform <sauce | browserstack | phantom > wouldn't be a better flag

[rase-]

I like the --platform flag. Unlikely confused with --browser-platform.

[rase-]

@yanatan16 any updates on this? Anything I can do to help? :)