Add DateTimeRange support(TSRANGE)

[razumeiko]

Hi! Thanks for the plugin!
I found some issue when i was trying to use TSRANGE data type. If you will try to use example from SqlAlchemy docs:

from psycopg2.extras import DateTimeRange
from sqlalchemy.dialects.postgresql import TSRANGE

class RoomBooking(Base):

    __tablename__ = 'room_booking'

    room = Column(Integer(), primary_key=True)
    during = Column(TSRANGE())

booking = RoomBooking(
    room=101,
    during=DateTimeRange(datetime(2013, 3, 23), None)
)

You will receive error saying "'during' is of type tsrange but expression is of type character varying"

So it's required to explicitly cast the value: during = cast(DateTimeRange(datetime(2013, 3, 23), None), TSRANGE).