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
Merge pull request #47 from aschrab/buildfix

Adjustments to building