Create new lesson 02 on forking

Create an empty lesson on forking and bump the other lesson numbers up accordingly.
add new extra lesson about forking

Add the first draft of a new lesson about forking. The lesson covers creating a fork, creating pull requests, and reasons for using a fork instead of working off of one repo. Leaves out the details of code review.

This encompasses most of the material that is in lesson 03. The plan is to rewrite 03 to focus on pull requests and code review. It should cover using pull requests from different branches in the same repo as well as between forks.
clarify early why a fork is necessary

to address comment #1 on pull request #517, this makes explicit that you won't
always be able to push directly to repositories other people have made.
rework and expand lesson 03 on code review

remove redundant curriculum with 02-forking, add some additional images to help guide creating a pull request, and add an abstract guide to performing code review
Merge branch 'improve-code-review' of github.com:lonnen/bc into lonnen-improve-code-review

Conflicts:
	novice/extras/02-review.md
Fixing a couple typos
Add lesson about forking on GitHub

Adds a new lesson about forking on GitHub
and revises the code review lesson.

Closes #517.