Rake task to backup your repo to a git repository

There's currently no automated method for backing up your project's source.  This change would enable the user to configure a separate remote that can be used to do just that.  Two rake tasks handle creating the git remote and adding, committing and pushing to it.

Three new options:
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* bk_remote - Your remote backup repo name `git remote add ...`
* bk_repo - Your remote backup repo url
* bk_branch - Your remote backup branch

Two new tasks under the `backup` namespace:
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* config - Creates the remote given the options listed in your Rakefile (bk_remote, bk_repo, bk_branch)

* push - Adds, commits and pushes any changes to your backup repository