Add example for TravisCI + Browserstack

[cch5ng]

I was able to add the {tunnel: 'BrowserStackTunnel'} to the intern.js config. but is there some way to encrypt the BrowserStack username and access key (as you could in Travis).

also if my .travis.yml is set up to run Intern tests, do I still need to put my BrowserStack credentials in the intern.js file? thanks!