closes #69

- `stepagent!` for FarmPlot that delegates to crop state stepagent!
  function

- `harvest!`, `sow!` functions work on a `FarmPlot`, and delegate to
  the respective methods acting on a `CropState`

This commit includes a `sed` pass over all files to change all
occurrences of the former spelling "Persephone" to "Persefone":
`sed -ie "s/Persephone/Persefone/" *`

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 ;-) )