Add ability to whitelist domains via .configure()

Related to [#25](https://github.com/rollbar/rollbar.js/issues/25).

I had to add a few large chunks of code to util.js that I found in [release/rollbar-1.0.0-rc.10.js](https://github.com/cfurrow/rollbar.js/commit/12ca38894bbc0df25e4373817d645b11996edf3a) @ 12ca3889. Wasn't sure how to interpret the code gone missing, but still referenced in the rest of the code. Can anyone shed some light on this?

Otherwise, code is open to scrutiny. Thanks!
For every test example, I believe we don't need to use the done callback since we aren't using asyncronous assertions.