add todo.txt
allow strings to be deserialized as entities (interpreting them as id)
remove unused ReflectivePropertiesTypeAdapterFactory
move all json serialization (gson) logic in serialization package
better status codes and error handling in EntityResourceDispatcher
* map expected exceptions to appropriate status codes with json body
* use "201 Created" for POST
* also add commons-logging as dependency for logging
implement AbstractEntity#equals and #hashCode()
* mainly to have "consistent" hash codes, e.g. for "consistent" HashSet ordering