Improvements on #87

This PR is the continued work on #85 after #87 is merged, and obsoletes #95 . This would close #85 .

### Code clean up

* extracted a function to post status for non-mergeable PR for reusing
* avoid infinite loop by: only post status when the state is external, i.e. not originated by the tooling server itself

### Experience change

The "Detail" link in #87 is confusing, because it presents texts encouraging a PR from upstream to the feature fork, but since it's not mergeable, creating a reversal PR doesn't make sense. I wish people to focus on the commits behind upstream, so I added an anchor to jump to the commit log in the comparison, skip the creating PR part to avoid confusion, see https://github.com/utensil/nupic.tools/commit/d01d19d9bb1cdb17430f61fec010ba6872f16a00#diff-d80ef43a14965bb127557b7a63337210R219

### What's different from #95 ?

It no longer handles the state change event, because it turns out that travis won't start for a non-mergeable PR, so no override of status would happen at all.