This commit removes the ability to reference a shared TradingEnvironment through the zipline.finance.trading module. In place, the classes that require a TradingEnvironment, or its child AssetFinder, contain their own references to those objects.
This commit also adds serialization utilities that allow for the pickling/unpickling of objects without unintentionally their TradingEnvironments or AssetFinders.
Previously the class SerializeableZiplineObject was used to
house basic __setstate__ and __getstate__ methods. It wasn't
really doing much that was helpful, so it is now gone.
jfkirk