Ability to accept incoming emails from unknown users (#2230, #3003).

Ability to accept incoming emails from unknown users (#2230, #3003).

An option lets you specify how to handle emails from unknown users:
* ignore: the email is ignored (previous and default behaviour)
* accept: the sender is considered as an anonymous user
* create: a user account is created (username/password are sent back to the user)

Permissions have to be consistent with the chosen option. Eg. if you choose 'create', the 'Non member' role must have the 'Add issues' permission so that an issue can be created by an unknown user via email. If you choose 'accept', the 'Anonymous' role must have this permission.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@2789 e93f8b46-1217-0410-a6f0-8f06a7374b81