Add install instructions

Added:
- mac instructions (assuming mac users will probably have 2.7, I said we'd provide extra if many have 2.6, since it will matter less without GAE. I'm happy to change that if we want to install the Python from the web site, but I figure it cuts down on work for everyone, and since we're not discussing virtual environments...)

- conceptual stuff at the beginning, about files/command line, a few notes, a few other edits and additions
- images for the mac things (can add or change as well)