greenthread: add .unlink() method; Thanks to Astrum Kuo

Compatibility with gevent.
https://github.com/eventlet/eventlet/pull/82
green: subprocess: do not use `new` module for compatibility with Python3

Module `new` was removed in python 3 so we should
remove it's usage due to py3 compability

https://github.com/eventlet/eventlet/pull/87
travis: Build with PyPy; Upgrade dependencies; Install libzmq-dev
pypy compatibility
WIP