Auto-increment patch names

Rather than naming all pull requests `prose-patch`, name it `prose-patch-N` where `N` is the lowest possible number not already in use.

This generates the patch name in `forkRepo` and must also retrieve the patch name (from the refs passed to the callback) in `sendPatch`.

See #257 for additional discussion on the problem.
Don't delete `prose-patch` branch after submitting patch, fixes #257

When a non-collaborator user clicks commit, prose silently forks the repo, creates a branch, submits a pull request, and deletes the branch. Deleting the branch triggers GitHub to close the pull request, making a subsequent merge impossible.

This patch removes the logic from `_includes/model.js` that deletes the branch following a pull request. However, further refinement of the patching workflow may be required (e.g., to create a new branch for each pull request to avoid smushing commits together).