rename composition to model internally. move Composition to separate file. don't pass mapper to #setup_fields anymore.
move creation of property accessors to Form::property.
Form#to_nested_hash now returns symbolized keys.
no longer delegate property accessors to the model but to @fields. this was a bug, when calling #validate the model should remain untouched, but we already set the attributes.
simplify coercion test.
make nesting almost dynamic ;)