Allow passing datetime objects to the parameters

[valo]

Trying to do this:

san.get("prices/bitcoin", from_date='2018-02-08', to_date=datetime.datetime.now(), interval="1d")

gives an error:

ParseError: Expecting a string datetime.datetime(2018, 7, 5, 17, 23, 46, 788937)

I think we can make the date time parameters accept datetime/date objects directly.