Move from distutils to setuptools

[RKrahl]

Move from distutils to setuptools as far as sensible. Close #4.

Note that with this change, the command classes defined by distutils-pytest are not silently injected into the distutils.command package anymore. They need to be explicitly listed in the cmdclass keyword argument to setup() now. Custom code needs to be changed from:

from distutils.core import setup
try:
    import distutils_pytest
except ImportError:
    pass

setup(
    name = "dummy",
    packages = ["dummy"],
)

to:

from distutils.core import setup
try:
    import distutils_pytest
    cmdclass = dict(distutils_pytest.cmdclass)
except (ImportError, AttributeError):
    cmdclass = dict()

setup(
    name = "dummy",
    packages = ["dummy"],
    cmdclass = cmdclass,
)