Add theme_comment_wrapper() and theme_username() from Zengine

There are 2 theme functions in Zengine that theme developers might find really useful.

The theme_comment_wrapper() used in Zengine adds a <h2>Comments</h2> before the comments on a node page.

And the theme_username() used in Zengine adds rel="nofollow" to the homepage link added by commentators.

I've added theme as commented-out example functions.