Merged features from redmine_archive_issue_categories

Merged features from redmine_archive_issue_categories
(https://github.com/tofi86/redmine_archive_issue_categories)

This enables categories to be archived and nested. Archiving a parent category will archive child categories as well. (They will not show up in issue creation or context menus).
Tested on 2.5.2; seems to work all right. *I am not a ruby programmer* :]