unit test keep-alive implementation, refs #49

Intern runs unit tests on the remote Selenium server by navigating to the client runner HTML page. No Selenium commands are issued after this initial call to remote.get() below, so Sauce Labs will assume failure if unit tests take longer than the specified "idle-timeout" Sauce capabilities parameter. 

This PR takes care of issuing a keep-alive heartbeat to ensure that Sauce Labs knows the session is still active.

For more info  on Sauce timeouts, see https://saucelabs.com/docs/additional-config.