Fix Jenkins build URL with multi-project

Fixes #46 (closed) and #43 (closed)

Expected behavior

With multiproject enabled, merge requests should display successful or failed build status. Given a GitLab Jenkins service URL of http://jenkins/job/test1, a GitLab project called test-project and branch ref of feature/ci_test_4 the correct build URL is

http://jenkins/job/test-project_feature_ci_test_5/scm/bySHA1/7c29b98573a9086737795ba8021f31da6b966ace

Observed behavior

The build page URL was incorrectly generated, causing an error connecting to the CI server. Similarly, documentation was not updated to include information about the multiproject feature and how to properly configure it.

Given the same information as above, the current URL GitLab generates is

http://jenkins/job/test1/_feature/ci_test_4/scm/bySHA1/7c29b98573a9086737795ba8021f31da6b966ace