Add Decimal validator

[lambacck]

Needs some work on docstring basically:

class Decimal(RangeValidator):
    """Convert a value to an integer.
    Example::
        >>> Decimal.to_python('10')
        10
        >>> Decimal.to_python('ten')
        Traceback (most recent call last):
            ...
        Invalid: Please enter an integer value
        >>> Int(min=5).to_python('6')
        6
        >>> Int(max=10).to_python('11')
        Traceback (most recent call last):
            ...
        Invalid: Please enter a number that is 10 or smaller
    """

    messages = dict(
        decimal=_('Please enter a decimal value'))

    def _convert_to_python(self, value, state):
        try:
            return decimal.Decimal(value)
        except (ValueError, TypeError):
            raise Invalid(self.message('decimal', state), value, state)

    _convert_from_python = _convert_to_python