Add requests transport (#66)

* Add requests transport
* Parametrize http_request so that both urllib3 and requests are exercised in system tests.
diff --git a/docs/conf.py b/docs/conf.py
index bd64578..cfd452f 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -366,7 +366,8 @@
 # Example configuration for intersphinx: refer to the Python standard library.
 intersphinx_mapping = {
     'python': ('https://docs.python.org/3.5', None),
-    'urllib3': ('https://urllib3.readthedocs.io/en/latest', None),
+    'urllib3': ('https://urllib3.readthedocs.io/en/stable', None),
+    'requests': ('http://docs.python-requests.org/en/stable', None),
 }
 
 # Autodoc config