added requires_openproject to Redmine::Plugin

- method to check OpenProject core version from the rails3 plugins
added deprecation warning for ...

- Redmine::Plugin.requires_chiliproject() and
- Redmine::Plugin.requires_redmine()
removed requires_chiliproject and requires_redmine

completely
No migrated plugin should use the old methods and
the rails2 versions are not compatible anyway
fixed plugin unit tests
Merge pull request #183 from opf/feature/rails3_add_requires_openproject

Add requires_openproject to Redmine::Plugin, remove redmine and chiliproject variants