Update for local documentation building.

I had to comment out the example intersphinx mapping as None is
apparently no longer a valid map value.

I also attempted to use extras_require to specify documentation
dependencies which are probably incomplete but it was enough to build on
my machine.

doc/conf.py

@@ -232,4 +232,4 @@
 
 
 # Example configuration for intersphinx: refer to the Python standard library.
-intersphinx_mapping = {'http://docs.python.org/': None}
+# intersphinx_mapping = {'http://docs.python.org/': None}

setup.py

@@ -22,6 +22,10 @@
             'flake8-quotes',
             'pytest',
         ],
+        'doc': [
+            'catkin_sphinx',
+            'sphinx'
+        ],
     },
     'author': 'Tully Foote, Ken Conley',
     'author_email': '[email protected]',