Support Python 2.7.6 by also adding a positional parameter 'to_close' to _execute_child.

In Python 2.7.6 released on November 10, 2013, the new parameter (containing file descriptors
to close) was added [1] to fix closing fds if exec'ing failed [2].

This was originally reported in [3].

[1] http://hg.python.org/cpython/rev/43749cb6bdbd
[2] http://bugs.python.org/issue18851
[3] https://bugzilla.redhat.com/show_bug.cgi?id=1040210
Implement _execute_child only once.
Adapat class once when class is created

There is no need to plug the correct _execute_child funcion when an
instance is created, since Python version does not change between calls.
Merge pull request #2 from nirs/compat

Adapt class once when class is created.
Merge pull request #13 from lenoch/master

Support Python 2.7.6 by also adding a positional parameter 'to_close' to _execute_child.