Optionally mark builds as unstable

For our UI tests we need another build result status when the UI tests fail: unstable. Some of our projects depend on remote backends which we don't manage. The current calabash-report plugin marks a build either as failed or as success. But when it fails, it could imply that our code broke something. With some of our projects, the most failures are related to the backend. That's why we need a different status when tests fail.

The changes in this repository fixes the issue described above. An extra checkbox is added in the advanced configuration section to enable this feature. The perform method now uses the hudson.model.Result for the build status instead of the return value of method CucumberReportPublisher.perform.

To finish, the documentation has also been updated.