Add default properties to agents
Add ability for agents to reproduce, + tests
BasicPlants inherit default props from Agent, and add new
Fix index.html again....
Safer removal of agent sprite
Move winter image to top of Sprite stack every rendering

This appears to be the correct/efficient way to deal with z-index
ordering, but we'll need to do a bit more research.
Initial work on seed-producing plants; update field model

More plant properties (flowering, etc.) need to be moved to
the BasicPlant class, so that we can refer to them directly
without expecting them to be in the species file.

Also we need to add environment resources to prevent over-crowding