import socket on windows

Should fix #459. See https://github.com/gevent/gevent/issues/459#issuecomment-49506870

Thanks to @snaury.

This reverts commit 12e4fb18cc731ee0dbbf0ebac8a20b4070268ea5.
update known_failures.py w.r.t PyPy upgrade

https://bugs.pypy.org/issue1743 is fixed now, but other failures appeared.
Merge pull request #505 from gevent/import_socket_win

import socket on windows
Merge pull request #506 from gevent/known_failures_pypy

update known_failures.py w.r.t PyPy upgrade
upgrade libev to 4.19