Fix a lurking GC-related crash with stat watchers on PyPy, detected by CPython re-using the memory much faster.

Make the CFFI backend handle the watcher's args property more like the Cython backend. Fixes test__core_watcher.

This allows building, using and testing corecffi on CPython, which can
make debugging easier. It also simplifies the build steps for PyPy.

Start testing corecffi on CPython too for comparison with PyPy and speed
comparisons with CPython. Currently getting a few unexpected failures
that need investigation; could be bugs in corecffi.

Remove some FLAKY tests from appveyor that should no longer be FLAKY; add test_selectors for 3.5/3.5 because it has timeout-related code that can be flaky there. Remove obsolete failure from PYPY case because we don't support that old version anymore.

test_hub_join_timeout was still flakey on Py 3.4/64-bit appveyor

One finer-grained skip for appveyor

AppVeyor: Make a number of timing-related tests less strict, hopefully fixing most of the appveyor failures

issues_671: Make more robust by sending a second signal and printing diagnostics.
Skip the few test cases that are still timing out unreliably on appveyor.
More explicit tests for gevent.Timeout functionality plus timing fixes.

Normalize recent GNU ls output to match the expected BSD-style output present in the docstrings of subprocess.py under Python 3. Fixes #681.

Clean up SSL exceptions and add tests for #317 and SSL timeouts in general

Additional tests for timeout/exception handling of ssl sockets; make behaviour consistent with stdlib (sendall does what python 3.5 does); fixes #317

Typo fix. Corrected spellings.

Use modern CFFI methods; drop unsupported PyPy versions < 2.6.1

No need to run fulltoxtest on PyPy---it doesn't work on Windows so the threaded file ops won't be used, and c-ares isn't a recommended configuration.

Give up on PyPy/Win for now due to the linking issues. Don't include libev_vfd if we don't need it. Update install.ps1.

Try make steps in a separate file on win and try to resolve link errors with an earlier setuptools import.

Use libev_vfd in corecffi for better Win32 compat (still not perfect). Fix a bug changing the fd and events of a io watcher and test it. Test pypy/windows.

This lets the CFFI backend be used on CPython and is more maintainable
going forward. Initial benchmarking of the CFFI backend on CPython
shows it to be a few percent slower than the cython backend, but
probably not significantly so.

Note that the tests are not expected to pass on Travis/pypy yet,
because Travis pypy is too old. Use of pyenv may be required.

Explicit test and documentation that fork-watchers and gevent.subprocess do not work in background threads. Fixes #688.