Added option to auto merge on build success

1. Provides option to Auto merge PR on successful build.
2. Updated github-api plugin as v1.55  to make use of PR merge functionality.
3. Merge pull request based on comment phrase from adminlist users
4. Provide custom merge message
A better configuration here would be that the "autoMergeSuccessfulPullRequests" checkbox enables the phase field; if the phase is blank, then it will always merge. If there is a phase, then there has to be a comment.  I could see people checking the box *and* entering a phase, and the checkbox wins.

Thoughts?