Added 'Value' validator

Value validator is useful if you need to check a field value over a list of allowed values.
Formatting is shitty, try sth like this:
```
raise ValidationError(
    ("Value '%s' is not an allowed value. It should be equal "
      "to one of the following values: '%s'.")
    % (value, ', '.join(self.allowed_values)))
```
Note that there is no `+` sign - it's not necessary inside of brackets. Consider also use of `.format()` since it can fail for numbers.