Plugin load sequence can now be specified via config.plugins

Hi all - there are some requests I found to make redmine load user plugins in a configurable sequence. Some advised to use config.plugins - settings (http://www.redmine.org/issues/12253, www.redmine.org/boards/2/topics/27473, http://theadmin.org/articles/redmine-plugin-dependencies/) but that never seemed to get actually implemented. I needed this for my plugins and changed the code which determines the plugin sequence. It is now minding the config.plugins setting as described in the contributions above. Cheers, Andreas