New Datetime Methods

Im not sure if I did schema_method_list and method_properties right (at the bottom of datetime.py). Please let me know. 

Deals with https://github.com/ContinuumIO/blaze/issues/712
I would do some research, maybe google around to see what is common in various SQL systems, Mongo, or other libraries.

Choosing the right term here is important and deserves consideration.