ENH: Adds a new `api_method` called `get_environment` so that users may
check if their algorithm is running in zipline or on Quantopian.
Merge pull request #384 from quantopian/q-or-zipline

get_environment api_method
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.