Fixes #29. Add support for merge request from forks
Update documentation for merge requests from forks
Merge remote-tracking branch 'upstream/master' into forked-merge-requests

Conflicts:
	README.md
	src/main/java/org/jenkinsci/plugins/gitlab/GitlabBuildTrigger.java
	src/main/java/org/jenkinsci/plugins/gitlab/GitlabBuilds.java
	src/main/java/org/jenkinsci/plugins/gitlab/GitlabCause.java
	src/main/java/org/jenkinsci/plugins/gitlab/GitlabMergeRequestWrapper.java
Merge pull request #35 from hrubi/forked-merge-requests

Fixes #29. Add support for merge request from forks