JENKINS-7824: Add 'assigned by' field and optional emails

Hi,
The attached PR builds on the changes for 7824 done so far to allow build failures to be assigned as well as claimed by:
 - recording who did the assignment, as well as who the build was assigned to
 - optionally sending an email to the assignee when an assignment is made. Whether emails are sent or not is a global configuration option, added to the Jenkins configuration window in a 'claim' section. (a future refinement might be making this configurable on a per project basis). If emails are to be sent, then there is an optional dependency on the mailer plugin.

Can you have a look and give me any comments, or merge if they seem a reasonable addition?

Ninian
Why not use [`MailAddressResolver.resolve(User)`](https://github.com/jenkinsci/mailer-plugin/blob/master/src/main/java/hudson/tasks/MailAddressResolver.java)?