merge to v1.5

[nutso]

• Backwards-compatibility to Redmine 2.2 by testing if issue.closed_on? method exists (Can't create recurring issue #49)
• Better documentation of plugin permissions
• Reversible migrations so can uninstall fully (impossible to uninstall, the database won't downgrade #53)
• Highlighting to user that adding recurrence via issues page is the best way (When adding new recurrence, closed issues are displaying in the listbox. #52)
• Stable support for both Redmine 2.x and Redmine 3.x (recur_tasks: NoMethodError: undefined method `default_status' #71); (Redmine 3.0.1 Support #67) and (Problem with 3.0.1 #69)