setup.py: Add tests_require.

Including `mock` and `nose` in the setup_requires list installs them
onto the production system which is not at all what we want.

Author: dave-shawley

setup.py

@@ -18,15 +18,18 @@
   "requests",
   "requests_oauthlib",
   "certifi",
+]
+tests_require = [
   "nose",
-  "mock"
+  "mock",
 ]
 include_package_data = True
 
 try:
     from setuptools import setup
     options["install_requires"] = install_requires
     options["include_package_data"] = include_package_data
+    options["tests_require"] = tests_require
     setup(**options)
 
 except ImportError: