Add support for merging after build.

- Adds the new command lander-merge-result
- Add a job to perform the merge via github api.
- Add a job to submit a comment on the pull request issue if it failed.
- Add support to the github api for marking a failing build, getting a single
pull request, and performing a merge.
Update readme with a note/warning
Merge pull request #5 from mitechie/add-lander

Add support for merging after build.