Allow doc string extraction and use for pluginapi.

Allow us to extract docstrings from our sources using the
sphinx.ext.autodoc module.

First use: Extract some of the docs for the pluginapi and
provide it in a new "Plugin API" section.

docs/source/conf.py

@@ -26,7 +26,7 @@ sys.path.insert(0, os.path.abspath(os.path.join('..', '..')))
 
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
-extensions = []
+extensions = ['sphinx.ext.autodoc']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['source/_templates']