Swap parents of final commit in intro/3 level goal

Change the order of parents of the final commit in the goal of the level
intro/3 to match the history produced by the expected solution. Although
either order is accepted the new, consistent order of parents is the
more idiomatic one merging a bugFix into master rather than master into
bugFix. Actually producing the old goal would require more commands.

Even though ordering of parents is too advanced of a topic to be covered
this early, it would be nice to keep that consistent.
Use relative URL for ?NODEMO

Change the NODEMO link to use a relative URL so that clicking the URL in
a development build will remain in that development build rather than
going to the main site.
Replace incorrect information about HEAD

Avoid stating that changing HEAD alters the working tree.  Although that
is a common next step they are not actually the same action and can be
done independently (e.g. `git reset` without `--hard`).

Also state that committing while on a branch only alters the branch,
without actually changing HEAD; HEAD only gets the new commit because it
points to the branch rather than a specific commit.
Merge pull request #77 from aschrab/work_tree

Remove incorrect information about HEAD
Better flip logic, visuals improvement, and also advanced level section
featuring Aaron's modifier tutorial

Merge pull request #80 from aschrab/multiple_parents
Merge branch 'master' into gh-pages