Start of new Git Intermediate lessons

This Introduction borrows heavily from characters, concepts, and prose
from git/beginner/00-intro.md by @gvwilson.
Added first part of Conversational Git section.

(Also, some cleanup to introduction, and added index)
fix line break typo
minor grammatical fixes, added BitBucket
2nd draft of Conversational Git

* Add objectives
* Revise clone and log sections
* Add checkout and reflog commands
Incorporate feedback from @wking

See:
https://github.com/swcarpentry/bc/pull/243#issuecomment-32733871
Fix code formatting
Fix code formatting
Newline missing for one code markup
Merge pull request #243 from ahmadia/git-intermediate

Git intermediate