Changed some stuff which have to do with views

* Added 'description' as a property of sponsors
* Added different view pages for the collection of groups (/groups/) and the collection of sponsors (/sponsors/)
* Fixed a bug in BeeHub_Registry which failed to load the collections of users, groups and sponsors
* Fixed a wrong name in the group view (field had name 'email' although it should be 'description')
* BeeHub::best_xhtml_type() now always returns 'text/html', because the client uses ExtJS, which doesn't support X(HT)ML documents
* Fixed a bug in BeeHub_View which tried to use an object attribute which could be left unset.