Add option to change row class about rows

[berinhard]

I'm love with this attrs lib! I'll try to take a chance on this feature sometime

[turicas]

We can currently override the row class by changing Table.Row attribute, like in this example:

import rows

class City:
# Could also be `class City(rows.BaseRow)` if a base class for `Row` is available

    def __init__(self, state, city, inhabitants, area):
        self.state = state
        self.city = city
        self.inhabitants = inhabitants
        self.area = area

    @property
    def density(self):
        return self.inhabitants / self.area

data = rows.import_from_csv('examples/data/brazilian-cities.csv')
data.Row = City
for row in data:
    print(row.state, row.city, row.density)