Implemented the functions needed for Agents.jl

This is a major commit, probably quite a bit too big...
However, all the pieces here depended on each other, so
I had to get it all working at the same time.

The model now includes:

	1. reading in map data
	2. creating an Agents.jl model object
	3. initialisation and stepping functions for the whole model
	4. a basic skeleton of nature.jl
	5. an example animal species (the Wolpertinger ;-) )