Add documentation, missing options and datetime support to expire commands

[ytkimirti]

Now expire and pexpire can be used with datetime.deltatime and
expireat and pexpireat can be used with datetime.datetime

import datetime

# Expire next month
date = datetime.datetime.now() + datetime.timedelta(days=30)
redis.expireat("mykey", date)

# Expire after a month
redis.expire("mykey", datetime.timedelta(days=30))
And they support the nx xx gt lt options