Add dynamic mail.debug support to JIRA

Currently to perform low level mail debugging you have to set mail.debug=true on the command line and restart JIRA.

We should be able to toggle a switch in the logging section of JIRA and have it turn on dynamically.

Ideally one should only have to set System.getProperties().setProperty*"mail.debug","true") however there is a block on this

In order to get this to work, atlassian-mail needs some surgery as decsribed here:

http://jira.atlassian.com/browse/MAIL-38

Also in the IMAP/POP fetcher we dont use the same mechanism but instead rely on javax.mail.Session.getDefaultInstance() which is a static. We need to call setDebug() on this object before using it.

After that we should be able to get dynamic mail debugging in place without a restart.