Adds User and Version custom field format that can be used to reference a project member or version in custom fields (#2096).

These new field formats are available for project, issue, version and time entry custom fields.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@5272 e93f8b46-1217-0410-a6f0-8f06a7374b81