ENH: Coerce user input with API method decorator

Previously we have capitalized input strings at different levels in
our code: in the user-facing API methods and in the asset finder.
This commit moves input string capitalization exclusively to the API
method to which the string was supplied. Specifically, the string is
capitalized by a preprocess API method decorator. The preprocess
decorator passes the input string to the newly defined
ensure_upper_case() method, which returns a TypeError if the argument
supplied is not a string.

ensure_upper_case() is defined in a new file, zipline/utils/input_validation.py.
The existing expect_types() method is also moved there.

Various tests in tests/test_assets.py are modified to account for the
fact that the asset finder method lookup_symol() no longer capitalizes
its supplied argument.

tests/test_algorithm.py

@@ -425,6 +425,23 @@ class TestMiscellaneousAPI(TestCase):
         self.assertIsInstance(algo.sid(3), Equity)
         self.assertIsInstance(algo.sid(4), Equity)
 
+        # Supplying a non-string argument to symbol()
+        # should result in a TypeError.
+        with self.assertRaises(TypeError):
+            algo.symbol(1)
+
+        with self.assertRaises(TypeError):
+            algo.symbol((1,))
+
+        with self.assertRaises(TypeError):
+            algo.symbol({1})
+
+        with self.assertRaises(TypeError):
+            algo.symbol([1])
+
+        with self.assertRaises(TypeError):
+            algo.symbol({'foo': 'bar'})
+
     def test_future_symbol(self):
         """ Tests the future_symbol API function.
         """
@@ -450,6 +467,23 @@ class TestMiscellaneousAPI(TestCase):
         with self.assertRaises(SymbolNotFound):
             algo.future_symbol('FOOBAR')
 
+        # Supplying a non-string argument to future_symbol()
+        # should result in a TypeError.
+        with self.assertRaises(TypeError):
+            algo.future_symbol(1)
+
+        with self.assertRaises(TypeError):
+            algo.future_symbol((1,))
+
+        with self.assertRaises(TypeError):
+            algo.future_symbol({1})
+
+        with self.assertRaises(TypeError):
+            algo.future_symbol([1])
+
+        with self.assertRaises(TypeError):
+            algo.future_symbol({'foo': 'bar'})
+
     def test_future_chain(self):
         """ Tests the future_chain API function.
         """
@@ -493,6 +527,23 @@ class TestMiscellaneousAPI(TestCase):
         with self.assertRaises(UnsupportedDatetimeFormat):
             algo.future_chain('CL', '2015-09-')
 
+        # Supplying a non-string argument to future_chain()
+        # should result in a TypeError.
+        with self.assertRaises(TypeError):
+            algo.future_chain(1)
+
+        with self.assertRaises(TypeError):
+            algo.future_chain((1,))
+
+        with self.assertRaises(TypeError):
+            algo.future_chain({1})
+
+        with self.assertRaises(TypeError):
+            algo.future_chain([1])
+
+        with self.assertRaises(TypeError):
+            algo.future_chain({'foo': 'bar'})
+
     def test_set_symbol_lookup_date(self):
         """
         Test the set_symbol_lookup_date API method.