AbstractMessageHandler patch (provides pluggable findUserByEmail, and external property file support for handler params + inbound issue creation fixes for Component,Priority,IssueType,Assignee)

As it stands, Jira's ability to identify a user based on their email is unusable in my environment. We have many users that have short ID's, eg "auser"and an email address of various formats such as "auser@company.com", "first.lastname@company.co.uk", "first.lastname@company.com". Any of these email accounts could be used to send mail to Jira, which doesn't help.

To fix this problem I've modified the Handler to make the lookup of USERID from EMAIL a pluggable feature, with implementation specified as a handler parameter. This lead to JRA-14954 which this issue also fixes.

I've written an Microsoft Active Directory 'plugin' to the handler which does all required cross-matching of email aliases to be able to find a valid sender, this makes use of an LDAPUtil library I did a while back for Confluence CSUM Plugin. The default behaviour is made available through this mechanism. The LDAP library is still in 'head' and needs versioning (next job) and was GPL3 but Ill fix that to be BSD - so who knows what may happen there.

I attach this fix in the hope that it will be integrated into the main code-base so I don't have to keep building it every new release, and others may find it useful.