Added 'Bulk edit' functionality.

This can be done by clicking on the edit link (little pen icon) at the upper-left corner of the issue list.
Most properties can be set (priority, assignee, category, fixed version, start and due dates, done ratio) and a note can be entered.
Only issues of the current project can be selected for bulk edit (subproject issues can't).

git-svn-id: http://redmine.rubyforge.org/svn/trunk@817 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/models/issue.rb

@@ -143,7 +143,7 @@ class Issue < ActiveRecord::Base
   
   # Users the issue can be assigned to
   def assignable_users
-    project.members.select {|m| m.role.assignable?}.collect {|m| m.user}
+    project.assignable_users
   end
   
   def spent_hours