Mantis importer:

* validation skip on issue assignement (assignee must be a project member in Redmine)
* Mantis bugs with severity 'feature' are imported as 'Feature request' in Redmine

git-svn-id: http://redmine.rubyforge.org/svn/trunk@660 e93f8b46-1217-0410-a6f0-8f06a7374b81