Add FractionField

Sometimes it's useful to represent fractions, instead of using float or decimal.Decimal. Python has the fractions module in the stdlib (both Python 2 and 3) and we could use it.

A draft implementation (needs to check some corner cases, add tests):

from fractions import Fraction

class FractionField(rows.fields.Field):

    TYPE = (Fraction, )

    @classmethod
    def deserialize(cls, value):
        value = str(value or '').strip()
        if not value:
            return None

        try:
            return Fraction(value)
        except ValueError:
            raise

    @classmethod
    def serialize(cls, value):
        if value is None:
            return ''
        else:
            return str(value)

Start by locating the existing rows.fields.Field implementation and the field tests, then compare their conventions with Python's fractions module and the draft FractionField shown here. Add coverage for normal values and corner cases, with completion shown by passing the field tests and correct Fraction serialization and deserialization.