new plants: jungle and birch trees, cactus.

Implemented Jungle and Birch trees, added Cactus, made land generation
to plants a bit more selective
merge 5415065edc6dcef32e63b1340ce313d754dda511

merge from boksee with the added trees, cactus, etc.
updated screenshot
Update README.md
readme update (2)
Merge commit '868c7e1bb3acee6982fbeff10e0e03b7e9db4494'

Conflicts:
	README.md
	TODO.md
	Thumbs.db
	blocks.py
	gui.py
	inventory.py
	main.py
	screenshot.jpg