Gateguard

Fixed
- raised `required` error if value is zero;

Fixed
- inheritance schemas;
- `validate_*` methods call;

Added
- method validators named `validate_FIELD_NAME` to `Schema`;

Added
- `stop_on_error` argument to `Schema.validate` method for stop validation if at least one error found;
- `code` attribute to `ValidationError` class;

Addded
- collect validated value from validators;

Updated
- `ArrayField` child errors;