ENH: Add symbol api function

A symbol() lookup feature was added to Quantopian.
By adding the same API function to zipline we can
make copy&pasting of a zipline algo to Quantopian
easier.

tests/test_algorithm.py

@@ -38,6 +38,7 @@ from zipline.test_algorithms import (TestRegisterTransformAlgorithm,
                                      handle_data_api,
                                      noop_algo,
                                      api_algo,
+                                     api_symbol_algo,
                                      call_all_order_methods,
                                      record_variables,
                                      record_float_magic
@@ -294,6 +295,10 @@ class TestAlgoScript(TestCase):
         algo = TradingAlgorithm(script=api_algo)
         algo.run(self.df)
 
+    def test_api_symbol(self):
+        algo = TradingAlgorithm(script=api_symbol_algo)
+        algo.run(self.df)
+
     def test_fixed_slippage(self):
         # verify order -> transaction -> portfolio position.
         # --------------