Fixed pipes leaking into child processes, closes #169.

Use FD_CLOEXEC on the r/w pipe as well.

O_CLOEXEC or pipe2 aren't available in most Python versions, so I didn't try to use those. The real fix will be done automagically in Python 3.4 via http://docs.python.org/3.4/whatsnew/3.4.html#whatsnew-pep-446.