Remove namespace package

As discussed here https://bitbucket.org/jespern/django-piston/issue/173/

Author: keithhackbarth

setup.py

@@ -20,7 +20,6 @@
     author = 'Jesper Noehr',
     author_email = '[email protected]',
     packages = find_packages(),
-    namespace_packages = ['piston'],
     include_package_data = True,
     zip_safe = False,
     classifiers = [