Action description: action instance in a context, one for all resource parameters

The return value of Action#prepare (nil or not nil) determines
if the resource has assigned a value or not. If there is no value,
resource parameter description does not contain a link to associated
resource, only null.
Action input/output: implement inheritance

Subclasses inherit I/O params from parent. All actions have defined
I/O, even if empty.
Paginable, includable module adding paging params to action input

Adds parameters limit (max number of entries returned) and offset
(offset of first returned object).
Parameters::Resource#clean: accept nil