Added documentation and simplified Grid initialization.

A couple of minor changes:
1. Added some error-checking to the Grid initialization process to
   catch invalid data.

2. Removed build_grid_from_src function and integrated it into the
   Grid.__init__ method.

3. Added comments explaining a couple of things in the program.
Refactoring, docs and other changes.

Made a few small changes.
1. Integrated build_grid_from_src into Grid class
2. Added comments in a few places
3. Added ability to customize game from command line.
	Example: python gol.py
	Running without any command line arguments will run the
	game as before with the default options.

	Example: python gol.py grid2.txt .5
	Running with arguments such as the above will run the
	game with source file "grid2.txt" (in the local
	directory) with a delay of .5 seconds)