object_api: support the number protocol

This commit revamps the object_api class so that it maps most C++
operators to their Python analogs. This makes it possible to, e.g.
perform arithmetic using a py::int_ or py::array.

tests/test_pytypes.py

@@ -1,3 +1,4 @@
+from __future__ import division
 import pytest
 import sys
 
@@ -238,3 +239,10 @@ def test_hash():
     assert m.hash_function(Hashable(42)) == 42
     with pytest.raises(TypeError):
         m.hash_function(Unhashable())
+
+
+def test_number_protocol():
+    for a, b in [(1, 1), (3, 5)]:
+        li = [a == b, a != b, a < b, a <= b, a > b, a >= b, a + b,
+              a - b, a * b, a / b, a | b, a & b, a ^ b, a >> b, a << b]
+        assert m.test_number_protocol(a, b) == li