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.
ENH: Well formed exception for any value passed to OrderStyle

This commit adds support for arbitrary objects in addition to NaN
and infinity values. The object well be returned in string format
as part of the error message.
Merge pull request #385 from quantopian/exception-on-nan-order-value

BUG: StopLimit better bad value exception handling
Added a utils file, which for now only includes datetime / month code conversions.
Merge branch 'master' of https://github.com/Quantopian/zipline