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.
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
02-collab: add retina screenshot for step 2

--HG--
extra : rebase_source : d5f1558ae44bb6f6eca3dc6aab27f34dde503c00
extra : histedit_source : 270c36cddb67b44b8332ed5f766dd16967e58209
02-collab: add retina screenshot for step 3

--HG--
rename : novice/hg/img/github-create-repo-03.png => novice/hg/img/bitbucket-create-repo-03.png
extra : histedit_source : 942e24ccf32d6f47ce397b1e186d35c3a8262883
02-collab: change git to hg after step 3
02-collab: change git svg to hg svg after step 3

--HG--
rename : novice/hg/img/git-freshly-made-github-repo.odg => novice/hg/img/hg-freshly-made-bitbucket-repo.odg
rename : novice/hg/img/git-freshly-made-github-repo.svg => novice/hg/img/hg-freshly-made-bitbucket-repo.svg
02-collab: add default path to .hg/hgrc

--HG--
rename : novice/hg/img/github-find-repo-string.png => novice/hg/img/bitbucket-find-repo-string.png
02-collab: add output of push command
02-collab: change git svg to hg svg after first push

--HG--
rename : novice/hg/img/github-repo-after-first-push.odg => novice/hg/img/bitbucket-repo-after-first-push.odg
rename : novice/hg/img/github-repo-after-first-push.svg => novice/hg/img/bitbucket-repo-after-first-push.svg