Add a Create or Comment regexp handler for incoming mail

As of JIRA 3.4, JIRA has a RegexCommentHandler, which parses emails for separators from broken mail clients (contributed by a user, IIRC). People now want the ability to create or comment on an issue, using regular expressions.

We should either:

• (preferably) pull the regexp parsing capability out of RegexCommentHandler, and add its 'splitregex' parameter to CreateOrCommentHandler.
• create a RegexCreateOrCommentHandler