Changed deprecated Hudson.getInstance() to Jenkins.getInstance().
Added more informative error message, if Jenkins instance is null. (Is there a work around?)
Merge branch 'master' of github.com:jenkinsci/poll-mailbox-trigger-plugin

Conflicts:
	src/main/java/org/jenkinsci/plugins/pollmailboxtrigger/PollMailboxTrigger.java
Changed deprecated Hudson.getInstance() to Jenkins.getInstance().
Added more informative error message, if Jenkins instance is null. (Is there a work around?)