Several fixes

fixes https://issues.jenkins-ci.org/browse/JENKINS-23483 and https://issues.jenkins-ci.org/browse/JENKINS-18110
The old RedmineManager which was included into the package was (afaics) a copy of the RedmineManager from redmine-java-api. As I could not see any reason, why not use the maven-dependency instead of a copy of the code, I removed the old one.