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
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
02-collab: remove mention of -u flag
02-collab: change git pull to hg pull
02-collab: change git clone to hg clone

--HG--
extra : amend_source : d4598c230ee50484595e7ce3176415db62905a79
02-collab: change git svg to hg svg after cloning

--HG--
rename : novice/hg/img/git-after-duplicate-clone.odg => novice/hg/img/hg-after-duplicate-clone.odg
rename : novice/hg/img/git-after-duplicate-clone.svg => novice/hg/img/hg-after-duplicate-clone.svg
02-collab: change git push to hg push