WIP: PyPy support (#527)

This commit includes modifications that are needed to get pybind11 to work with PyPy. The full test suite compiles and runs except for a last few functions that are commented out (due to problems in PyPy that were reported on the PyPy bugtracker).

Two somewhat intrusive changes were needed to make it possible: two new tags ``py::buffer_protocol()`` and ``py::metaclass()`` must now be specified to the ``class_`` constructor if the class uses the buffer protocol and/or requires a metaclass (e.g. for static properties).

Note that this is only for the PyPy version based on Python 2.7 for now. When the PyPy 3.x has caught up in terms of cpyext compliance, a PyPy 3.x patch will follow.
diff --git a/tests/test_issues.py b/tests/test_issues.py
index 2098ff8..e60b5ca 100644
--- a/tests/test_issues.py
+++ b/tests/test_issues.py
@@ -1,5 +1,4 @@
 import pytest
-import gc
 from pybind11_tests import ConstructorStats
 
 
@@ -55,7 +54,7 @@
     el = ElementList()
     for i in range(10):
         el.add(ElementA(i))
-    gc.collect()
+    pytest.gc_collect()
     for i, v in enumerate(el.get()):
         assert i == v.value()
 
@@ -130,13 +129,13 @@
     assert c.b.a.as_base().value == 42
 
     del c
-    gc.collect()
+    pytest.gc_collect()
     del a  # Should't delete while abase is still alive
-    gc.collect()
+    pytest.gc_collect()
 
     assert abase.value == 42
     del abase, b
-    gc.collect()
+    pytest.gc_collect()
 
 
 def test_move_fallback():