Add Jasmine unit testing framework

I added and configured the Jasmine unit testing framework which can be used to start writing some unit tests. Before writing any really useful tests, the Chrome API will probably need to be mocked. There's just enough in `tests/spec/chrome.helper.js` that the Jasmine framework can run without any errors.
Would be nice to replace this with that header test you mentioned, since you're committing, merging, etc.