If setuptools is available, use entry points for the pss executable rather than a hand-crafted script.

This is useful on Windows, where the scripts are not as robust as using an exe wrapper. The patch only uses entry points if setuptools is available at build time.

If a wheel is generated, and pip 1.5 is used to install the wheel, the executable wrapper does not depend on setuptools. If an install from source is done, the wrappers generated by setuptools rely on pkg_resources at runtime (but in this case, the fact that the install used setuptools means that setuptools *will* be available at runtime). This latter is a setuptools limitation.