Limit python setup.py test to our tests

Tell unittest.discover specifically what to look for instead of letting
them scan the entirety of our directory (which could have other
dependencies downloaded with their tests that will fail).

Closes #403

setup.py

@@ -30,6 +30,7 @@
          'Programming Language :: Python :: 2.7',
          'Programming Language :: Python :: 3.4',
     ],
+    test_suite="test_httpbin",
     packages=find_packages(),
     include_package_data = True, # include files listed in MANIFEST.in
     install_requires=[