run_all_test_files event

A potential implementation for #50.

> A flag is passed from the driver to the engine to identify whether a
> series of test files were queued as a result of the user triggering a
> run of all the test files.

To accomplish this, I added a flag to `Engine#run_test_files` that states whether it was triggered by the *run_all_test_files* event. This flag is passed in `Driver#run_all_test_files`.

It feels somewhat awkward and highly specific, but I can't think of any other way of being able to trigger this event, as the running of test files is abstracted in the Driver class (which makes perfect sense).

So, it's a working solution, but one that you may or may not think could be improved upon.