Backport of the subprocess module from Python 3.2/3.3 for use on 2.x.

This is a backport of the subprocess standard library module from Python
3.2 & 3.3 for use on Python 2.4, 2.5, 2.6 and 2.7. It includes bugfixes
and new features. On POSIX systems it is guaranteed to be reliable when
used in threaded applications. Bonus: It includes timeout support from
Python 3.3.