Experimental/WIP PyPy support #248

Some tests fail, consult known_failures.py for the list.

stdlib's signal module is not supported. Depends on https://bitbucket.org/cffi/cffi/issue/152/handling-errors-from-signal-handlers-in

On PyPy, sys.exc_info is shared between greenlets apparently https://bugs.pypy.org/issue1743.

based on gevent-on-pypy/pypycore@f04a41c written by Ralf Schmitt and Lucas Clemente Vella with patches from hasenj and Armin Rigo