TDD refactoring of library
ListOf validator added
call validator object instead of assign to raw_value in order to perform conversion
rename bind() to __call__ in UnboundValidator

Now there is no need to check if this is UnboundValidator object or Validator
class object in order to create bound validator instance - just
validator(owner=foo) can be used now in both cases.
ListOf validator rewritten
add length range test for ListOf validator
invoke inner validator's validation
add owner property to validator
add read only validator, args and kwargs props to UnboundValidator
get rid of errors_container_type - always use list
create formify/validators module and move validators.py there
remove unnecessary python_type from String validator
create LengthValidationMixin
create ValueValidationMixin and use it in Numeric validator
use process() instead of __call__ when using from internals
add missing super() statement
rename mixin classes
implementation and tests of Map validator
create tests for validator mixins
use standalone validators in tests