Copy git lesson files as basis for hg lesson.
Port index and intro pages to hg.
Delete Git staging area image that has no Mercurial counterpart.
Delete image re: Git HEAD~* mapping to rev ids; unneeded for Mercurial.
Port the version control "A Better Kind of Backup" section to Mercurial.
Fix typos re: review by @r-gaia-cs.
Initial lesson about python string formatting using the format() method
Removed solutions
Clarified the note about field widths and added an example
Clarified the sentence about the print command output
Fixed typos
Changed line width to 80 characters for the poem formatting and added an explanation
Added machine-readable vs human-readable output, and updated the challenges
02-collab: reorder git and mercurial words
Add materials for python errors lesson
Add "case sensitive" and "traceback" to glossary
Finish lesson with item errors, key points
Add cell tags for challenges
Add markdown file for errors lesson
02-collab: add retina screenshot for step 1

--HG--
rename : novice/hg/img/github-create-repo-01.png => novice/hg/img/bitbucket-create-repo-01.png
extra : amend_source : 5bb529b2449de7e9af5e0c6ed60a80f5b541e35f
extra : histedit_source : 00cbb7270625a1ab0f57aead314d1400b557610a