Bringing master branch's README.md into line with gh-pages branch's.
readme details
Merge pull request #1 from ptone/readme-improvements

Thanks.
Merging changes to README.md from ec153e80
Merging changes to README.md from gh-pages
Importing SQL cheat sheet
Importing Git lessons - but getting a weird build error (and  is rendered as an en-dash)
Importing legacy shell lesson
Removing summary section from lesson (was supposed to be in separate file)
Importing invasion percolation example: getting Jekyll/Maruku build errors, but no line numbers to help fix them.
Merge pull request #2 from gvwilson/gh-pages

Importing lesson material.
Explaining invasion percolation in lesson index
Trying to fix formatting errors
Importing sets and dictionaries material
Updating README to reflect actual structure
Renaming shell and git tutorials to allow for others beside them
1. Importing Python debugging tutorial from Anthony and Patrick (via the 'restructured' branch in the old repo).
2. Getting build errors in lessons/python-debugging/tutorial.md, but can't diagnose (no line numbers in error messages when run in Jekyll, and Maruku seems to stumble on Pygments highlighting).
Importing more material from The Hacker Within
Switching to redcarpet for rendering: let's see how this works on GitHub
Removing rusty instructions from README