`hub merge` should use GitHub's merge-button API

The `hub merge` functionality implemented in #164 creates the merge commit locally, but leaves the pull request open.  It would be better to use GitHub's [merge-button API][api] directly.  I'm agnostic on whether the commit created using the merge-button API should be fetched and merged locally after it is created on GitHub.

[api]: http://developer.github.com/v3/pulls/#merge-a-pull-request-merge-buttontrade