Fix property mapping between the DTO and entity

* CreateDropDTO uses java.lang.String for the drop image while
GetDropDTO uses  a GetMediaDTO. Therefore, each of these DTOs
(CreateDropDTO and GetDropDTO) needs a mapping entry in the dozer
config file.
Custom converters for rule conditions and actions

* The Jackson mapper sees the the List of action and conditions
for  rule as a list of objects. Therefore, intermedia objects are required
in (Get/Create)RuleDTO so that the actions and conditions are properly
deserialized and posted to the database