Fix #637: nose install fails w/ traceback using python 3.3.0

To cope with multiprocessing, nose attempts to rewrite some scripts.  It
turns out that some of the "scripts" are actually executables.  Under
Python 2.x, we'd just perform the substitution on the executable too,
but it never matched anything, and therefore, never replaced any bytes.
Under Python 3, that tactic no longer works because the scripts come as
a string, while the executables come as bytes.  We shouldn't try to
rewrite an executable anyways, so let's just skip them.