401. Plugin infrastructure

* implements installing, loading and setup for plugins
* codifies configuration
* has a sample plugin
* docs
* tests

mediagoblin.ini

@@ -30,3 +30,8 @@ base_url = /mgoblin_media/
 
 [celery]
 # Put celery stuff here
+
+# place plugins here---each in their own subsection of [plugins]. see
+# documentation for details.
+#[plugins]
+