BUG: Exception will be raised when nan value sent to order()

Previously order was not checking for nan values sent as
limit or stop prices. It will now raise a runtime exception
in the event that an attempt to order with a nan price is made.
Not sure if this is worth handling, but this will barf if the input object doesn't have a `__str__` or if its `__str__` or `__repr__` changes.  Might be slightly safer as:

`msg="""Attempted to place a {} order with a price of type {}.""".format(label, type(price))`