Added global option to never use merge commit

Restored after some fixes #47.

I think it should be possible to ignore MERGE commits and always checkout to HEAD in pull request build, so if there are some failing unrelated tests in upstream, they do not affect pull requests build results.

It is implemented in the same way `autoCloseFailedPullRequests` is and this is actually the first time I write Java :)

Also, there are some places where plugin checks if pull request can be merged (e.g. when updated Status API), but I am not sure what if the best way to use this option there.

I created no tests as my Java is bad, but I'm ready to add them with some guidance.

This is probably unrelated, but I think docs should be updated, because adding `${sha1}` as parameter in Git plugin configuration is not necessary. In our case, docs were misleading as we thought setting Git plugin to checkout to `${ghprbActualCommit}` would make the deal, but it was still checking out to merge commit.