Reorganize existing cheat sheets into topic subdirectories

This will keep things from getting too cluttered as the workshop
information fills in.
README.md: Line wrapping and reference-style links

This makes the README easier to read in a terminal or editor, while
leaving the Markdown output unchanged.
Merge pull request #10 from wking/readme-cleanup

README.md: Line wrapping and reference-style links
Merge pull request #8 from wking/cheat-sheet-structure

Reorganize existing cheat sheets into topic subdirectories
Replacing double quoted strings with single quotes for consistency across SQL implementations
Merge pull request #14 from ethanwhite/sql-quote-cleanup

Replacing double quoted strings with single quotes in SQL Cheat Sheet
Merge branch 'master' of github.com:swcarpentry/boot-camps