Separating Listener, and making it event-based.

Also made a test-suite for it.

@jjackson26 @swarajban Take a look.  The code was moved from main.js to listener.js mostly untouched, except for some refactoring to make it event-based, and to separate out the _initializeRecognizer logic.  With the spec and tests, we should be able to make changes more confidently.

There's more I'd like to put in the test suite, but it's not worth holding up the rest of the bugs for.
Not technically a duplicate.  #38 tests that `emit` is captured by `on`.  This tests that `listener.start` eventually leads to the `start` event.