Add getParent method for Commit objects

Add method to retrieve a given parent (not ancestor) of a commit object.
If the commit doesn't have the requested parent return null.
Improve parsing of relative references

Support parsing of references like HEAD~^2~3^ .  Previously, ~ required
a following number, ^ didn't allow a following number, and chaining
wasn't supported.

If at any point in the resolving of the chain the requested parent isn't
found report the error by specifying the id of the last commit that was
found along with the parent specification to help the user to figure out
where the problem arose.
Remove obsolete numBackFrom method

The numBackFrom method is no longer called from anywhere with the
changes from the previous commit to change how relative references are
handled.
Rebuild for changes to handling of relative refs
Fix capitalization of sandboxCommands file

On case-sensitive file systems, need to use correct case in require()
calls.
Fix path to levels from level arbiter

Need to go up to directories from the level arbiter to get to the
directory containing level definitions.
Add levels symlink at top level

Without this link running `grunt build` gives me errors like:

    Running "browserify:build/bundle.js" (browserify) task
    Fatal error: Cannot find module: "../../levels/intro/1" from directory "/home/ats/vc/learnGitBranching/src/levels" while processing file /home/ats/vc/learnGitBranching/src/levels/index.js
Target branch can't be second argument to merge

Remove instances in tutorial where the target branch for a merge is
supplied as a second argument to `git merge`.  The target branch for
merge is always the current branch.
No multiple arguments for merge

`git merge` should only be given multiple arguments if doing an octopus
merge which isn't currently supported.  Require that exactly one
argument be given, and always use HEAD as the currentLocation.
Merge pull request #47 from aschrab/buildfix

Adjustments to building
Merge pull request #49 from aschrab/merge-args

Fix merge argument handling
rebuild