Add support for non-anonymous supportclients.

Introduced the 'treat as supportclient' permission. If the user has this
permission, their incoming emails are expected to be support requests and
the 'owner-email' attribute is filled in the issue.
The  'treat as supportclient' is added to the 'Anonymous' role by default
(can be removed to avoid receiving support from unknown emails).
Add issue matching based on standard MIME header references

Standard Redmine notifications encode the issue reference into the Message-Id
and References headers and the incoming email handling mechanism parses
it. This is now added to the helpdesk mailer to be consistent with the
standard Redmine mailer.
Merge pull request #49 from orchitech/feature2-email-references

Add issue matching based on standard MIME header references
fix merge conflict
Merge branch 'orchitech-feature1-permission'
test with Redmine 2.5.x was successful #39 #40
bump version