Added error handling in validator for FA relations

For 1:1 relations FormAlchemy modifies the fieldname of the rendered field the
the name of the Foreignkey which is references another object. This causes
problems in the current validation code. First solution is now to just log the
warning for this case, skip any validation and store the raw submitted value
into the data dictionary.
Fixed logic on initialization of fields.
Fixed tests on unknown fields.
Added readonly flag to forms.
Provide readonly flag from config when configuring the field.
Set readonly flag in customform.
Propagate the readonly flag of form to the field

If the readonly flag is set for the form set all included field readonly too.
Adapted and added testcases.