Adding pre-commit and post-checkout hooks

* Pre-commit hook lints JSON
* Post-checkout hook causes hooks directory to update with current
  project hooks
* Readme updated so installation section reminds you to install hooks
Improved directory management in git hooks

* Make git hooks always execute utilities relative to git repository root
* And make them fail if called from outside the git repository
* Make utilities save current directory and return after doing their
  things
Merge pull request #7 from edx/jrbl/feature/json_lint_hooks

Adding pre-commit and post-checkout hooks