Added event hatch_starting for pre-simulation setup.

One of the use cases I wanted to use Locust for required that I do some initial setup before beginning the simulation. Two or more Locust classes needed to have knowledge of how many instances of another type of Locust class and that number could not be gathered from the hatch_complete event. So I added an event that fires right when STATE_HATCHING is entered so that initial setup could be done before running the simulation.