Add a pre-commit hook for jshint + moved coverage script

We noticed a lot of commits on master that were just running `grunt jshint` and fixing the resulting issues. It seems like it would be nice to use a pre-commit hook to ensure that commits are clean before they are pushed and a pull request is sent.

As part of this, we created a /scripts directory, and it seemed like a good idea to move the coverage merging script in there. The coverage directory is now gitignore'd.