Trying to use BrowserStack.

1. Add new intern.browerstack.js file, which may eventually replace intern.js
   (which connects to SauceLabs).   For now, run it via:

$ grunt test:browserstack

after setting up BrowserStack account, and BROWSERSTACK_USERNAME and BROWSERSTACK_ACCESS_KEY
environment variables.

2. upgrade to intern 2.0.3 (and thus leadfoot 2.0.2)

3. add node_modules to excludeInstrumentation list