Fix #208 - Prevent grunt publish from destroying the gh-pages branch

Getting the build process to keep `dist/filer-test.js` on `gh-pages` only without blowing away the branch was complicated by having to generate the file on a branch that didn't track it. As a result, here's what I had to do:

1. Apply a banner to the generated file in order to ensure it was unique per filer version (even when tests weren't changed or updated)
2. Add a number of other grunt tasks to stash the generated file on `develop`, and then pop and commit it on `gh-pages`.
3. Patch the `grunt-git` module to include a `gitrm` task. The followup issue to replace the reference to my branch with the latest official version of the module is here: #321