Flags to launch individual browsers from command line

Following the discussion in #59, is this what you had in mind for the flags, @defunctzombie?

The PR adds the ability to overwrite the browsers in `.zuul.yml` like this:
`zuul --browser-name chrome --browser-version 29 --browser-platform="Windows 2008" test/index.js`. The platform flag is optional.

Also added the possibility to specify the server script used with a `--server` flag, which might come in handy in travis setups.