Added cryptography section
Second revision.

- Added several additional links to wikipedia and to cryptography papers
- Re-stylized Markdown to use link references (so we aren't inlining tons
  of URLs)
- Edit document to use implicit link references where possible to further
  simplify/prettify plaintext version of document.
- Change some existing URLs to use HTTPS rather than HTTP
Merge pull request #9 from dwaite/master

Added cryptography section