Add provision to delete branch

[NikitashP]

motivation

• github enterprise being used at the moment does not give provision: https://help.github.com/en/github/administering-a-repository/managing-the-automatic-deletion-of-branches
• I want to be able to delete the branch after the PR has been merged

Changes

• https://developer.github.com/v3/pulls/#get-a-single-pull-request response is populated in org.eclipse.egit.github.core.PullRequest
• response has reference to branch in Object org.eclipse.egit.github.core.PullRequest#head - org.eclipse.egit.github.core.PullRequestMarker#ref
• this reference is needed to perform deletion : https://developer.github.com/v3/git/refs/#delete-a-reference

[NikitashP]

@aaronjwhiteside Thanks for Merging the PR. Do you have any idea as to when you will make a release with this commit? I would request a minor version Increment if possible.