added descriptive exception when document type exists in db but not in code
fixing backward compatibility: restored GetPropertyValue methods (https://github.com/spopovic/uSiteBuilder/pull/22#issuecomment-62870483)
introduced MixinBase to be able to add/modify logic there later w/o affecting client code; common properties from MixinBase and DocumentTypeBase moved to a class ContentTypeBase
Mixins are now proxies and virtual properties are lazy-loaded as expected