Make massive amount of improvements to Podcast's API Documentation

Additionally, rename earlier forgotten mentions of
feedgen to podgen, namely in podgen.Category's example
and the exclude_feedgen argument in Podcast.set_generator.

Author: tobinus

podgen/category.py

@@ -12,7 +12,7 @@ class Category(object):
 
     Example::
 
-        >>> from podgen.category import Category
+        >>> from podgen import Category
         >>> c = Category("Music")
         >>> c.category
         Music