Wraps the parts of the branch protection API that allows the user to require signed commits on a branch.
New methods are :
* Branch.get_required_signatures : returns true if signed commits are required
* Branch.add_required_signatures : adds the requirements for signed commits on a branch
* Branch.remove_required_signatures : removes the requirement for signed commits on a branch
It uses the API provided on this page : https://developer.github.com/v3/repos/branches/
Since this feature is in [preview](https://developer.github.com/changes/2018-02-22-protected-branches-required-signatures/) I pass the 'Accept application/vnd.github.zzzax-preview+json' header to each request.