JENKINS-11390 fix - Don't return null from getMsg()

If the user makes an Accurev transaction without a comment, then
AccurevTransaction::getMsg() returns null. The impact of this is
breaking plugins (e.g. email-ext) that rely on it being a non-null
value.

Per documentation on ChangeLogSet.Entry's getMsg() method, the return
value can be empty but never null.
Jenkins-11391 fix - Accurev changelog data not visible through Remote API

Added annotations to expose class/methods so that change data
available.  This follow guidelines in:
https://wiki.jenkins-ci.org/display/JENKINS/Change+log
Merge pull request #6 from jglouie/master

Jenkins-11390, 11391 fixes