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.

README.md

@@ -25,7 +25,7 @@ become an excessively large and unnecessary dependency.
 Think of this library as a tiny self-contained version of Boost.Python with
 everything stripped away that isn't relevant for binding generation. Without
 comments, the core header files only require ~2.5K lines of code and depend on
-Python (2.7 or 3.x) and the C++ standard library. This compact implementation
+Python (2.7 or 3.x, or PyPy2.7 >= 5.5) and the C++ standard library. This compact implementation
 was possible thanks to some of the new C++11 language features (specifically:
 tuples, lambda functions and variadic templates). Since its creation, this
 library has grown beyond Boost.Python in many ways, leading to dramatically
@@ -58,12 +58,15 @@ pybind11 can map the following core C++ features to Python
 ## Goodies
 In addition to the core functionality, pybind11 provides some extra goodies:
 
-- pybind11 uses C++11 move constructors and move assignment operators whenever
-  possible to efficiently transfer custom data types.
+- Python 2.7, 3.x, and PyPy (PyPy2.7 >= 5.5) are supported with an
+  implementation-agnostic interface.
 
 - It is possible to bind C++11 lambda functions with captured variables. The
   lambda capture data is stored inside the resulting Python function object.
 
+- pybind11 uses C++11 move constructors and move assignment operators whenever
+  possible to efficiently transfer custom data types.
+
 - It's easy to expose the internal storage of custom data types through
   Pythons' buffer protocols. This is handy e.g. for fast conversion between
   C++ matrix classes like Eigen and NumPy without expensive copy operations.
@@ -100,7 +103,7 @@ In addition to the core functionality, pybind11 provides some extra goodies:
 
 ## About
 
-This project was created by [Wenzel Jakob](https://www.mitsuba-renderer.org/~wenzel/).
+This project was created by [Wenzel Jakob](http://rgl.epfl.ch/people/wjakob).
 Significant features and/or improvements to the code were contributed by
 Jonas Adler,
 Sylvain Corlay,