change description in setup.py to match that of README.rst

Author: srittau

setup.py

@@ -6,7 +6,7 @@
 setup(
     name="asserts",
     version="0.6",
-    description="Rich Assertions",
+    description="Stand-alone Assertions",
     author="Sebastian Rittau",
     author_email="[email protected]",
     url="https://github.com/srittau/python-asserts",