add ISBN validator
replace String(multiline=True) with Text validator
move core validators from  __init__.py to core.py
major changes in internal API

* use Validator static API to check and get bound validators
* all parameters of Validator subclasses are passed to constructor
* deletion of attribute is not allowed in case of Validator properties
* add another kind of validators: grouping validators
* create SchemaVisitor to convert validators into fields
properly dispatch submitted data to grouped fields
remove field's key attribute (no longer needed)
grouping validators UTs fix