Add --set-commit-status flag to merge command

Setting this flag with create a success or failure status on the
latest commit of all PRs in the merge, depending if all PRs could
be merged or not.
Add new set-commit-status command

Based on merge, this uses the same filters etc. to find all relevant PRs
and then sets the given commit status on those PRs.
Add create_status method to PullRequest
Move code common to merge and set-commit-status commands to new base class
Restore original merge log message
Add PR candidate output to set-commit-status --info
Merge pull request #86 from knabar/commit-status

Add support for Github commit status