Port tests to pytest

Use simple asserts and pytest's powerful introspection to make testing
simpler. This merges the old .py/.ref file pairs into simple .py files
where the expected values are right next to the code being tested.

This commit does not touch the C++ part of the code and replicates the
Python tests exactly like the old .ref-file-based approach.

tests/test_pickling.py

@@ -0,0 +1,18 @@
+try:
+    import cPickle as pickle  # Use cPickle on Python 2.7
+except ImportError:
+    import pickle
+
+from pybind11_tests import Pickleable
+
+
+def test_roundtrip():
+    p = Pickleable("test_value")
+    p.setExtra1(15)
+    p.setExtra2(48)
+
+    data = pickle.dumps(p, 2)  # Must use pickle protocol >= 2
+    p2 = pickle.loads(data)
+    assert p2.value() == p.value()
+    assert p2.extra1() == p.extra1()
+    assert p2.extra2() == p.extra2()