MAINT: Removes the ability to reference a global TradingEnvironment

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.

tests/test_examples.py

@@ -31,8 +31,6 @@ from zipline.utils import parse_args, run_pipeline
 # Otherwise the next line sometimes complains about being run too late.
 _multiprocess_can_split_ = False
 
-from zipline.finance import trading
-
 matplotlib.use('Agg')
 
 
@@ -47,8 +45,6 @@ class ExamplesTests(TestCase):
     @parameterized.expand(((os.path.basename(f).replace('.', '_'), f) for f in
                            glob.glob(os.path.join(example_dir(), '*.py'))))
     def test_example(self, name, example):
-        # Create a new trading environment for each test.
-        trading.environment = trading.TradingEnvironment()
         imp.load_source('__main__', os.path.basename(example), open(example))
 
     # Test algorithm as if scripts/run_algo.py is being used.