Rewrite gen docs to make coroutine the primary decorator.
Update chat demo to be more exemplary.

Use gen.coroutine in auth handler (but not in wait_for_messages
because it makes things less clear and we don't have good cancellation
support).  No more mixin.  Python 3 compatible.
Fix most of the rest of the dangling references.
Fix docstring for locale.load_translations.

When unicode strings started using the u() function, this string
was no longer considered a docstring.
Fix dangling references in old release notes.