Add setOrCreateTextParameterValue

To share logic with StringParameterValue, add setOrCreateParameterValue.
Add option for selecting encoded string for multiline text parameters or not.
Add test for readableMessage
Fix some issues

* Increase IGNORE line number for checkstyle
* Missing import
Fix NPE for ChangeIdAnnotator

Check whether gerritCause.getEvent() returns null or not.

Fix for JENKINS-22634
https://issues.jenkins-ci.org/browse/JENKINS-22634
Fix slash in URL handling in REST connection test.

Fix for JENKINS-22595
https://issues.jenkins-ci.org/browse/JENKINS-22595
Merge pull request #144 from rinrinne/annotate

Fix NPE for ChangeIdAnnotator
Merge pull request #145 from rinrinne/rest-test-connection

Fix slash in URL handling in REST connection test.
Merge pull request #141 from rinrinne/freetext

Store plain free text to GERRIT_CHANGE_COMMIT_MESSAGE