Prepare for RSS by renaming terms

This library will soon expand to support RSS feeds.  Those feeds have a
different internal structure than Atom feeds.  To distinguish between
the two, I've renamed all terms carrying Atom information.  The term
names start with "atom_".

I thought that link/1 terms could be shared between RSS and Atom, but
they also appear to have completely different internal structure.
author/1 terms are similar.
Support RSS feeds
Fix mistake in summary/2 for Atom feeds
Release v0.1.0