first draft of SSH keys for GitHub lesson
remove OSX specific pbcopy, remove passphrase material, explain random art image
Merge pull request #470 from brenna/github-ssh-lesson

setting up SSH keys for GitHub lesson
Adding links to reference material on appropriate pages
Merge pull request #560 from gvwilson/adding-links-to-reference-guides

Adding links to reference material on appropriate pages
Add CSS for printing

The CSS was add at our main CSS and remove from printing:

- Logo at header
- GitHub ribbon
- Footnote

Close #561.
Shrink the font size for print version
Merge pull request #564 from danmaclean/creating-lesson-on-regular-expressions

Creating lesson on regular expressions
Merge pull request #562 from r-gaia-cs/media-print

Add CSS for printing
Merge branch 'master' into creating-lesson-on-regular-expressions