travis: Build with PyPy
travis: Fix "Invocation failed" when building with PyPy

Example: https://travis-ci.org/jstasiak/eventlet/jobs/22062267
pypy compat: Implement SocketDuckForFd's drop/reuse
pypy compat: Fix kqueue hub

PyPy (at least 2.2.x) raises IOError instead of OSError
tests: Make greenthread tests clean after themselves
pypy compat: Fix ssl wrap_socket test

Used to fail with "error: [Errno 9] Bad file descriptor"
pypy compat: Fix various tests
pypy compat: Fix greenio test_del_closes_socket test
pypy compat: Fix greenio tests
pypy compat: Fix refcount tests