Use Factory pattern for add attributes to criteria

Instead of every resource defining what attributes are extracted from a
SelectEntity, a Factory will do this. This reduces the code in the
resources and the information about how to extract attributes can be used
accrossed the whole project. This will be usefull for followning addition
to the API.
onGet returns now a Response instead of a list

This simplify the access to it. Also it returns now the Status Code 204 -
No Content when the list is empty.
Implement support for adding tasks to a tasksheet

Implement support for new interface for 1 to m relations, the
ConnectWithEntity class.