added/changed error message

Added error message if RSS settings are for excerpt instead of full text, since plugin won't work.
Simplify/Alternate conflict error message now only shows on Media Settings page.
added condition for early return to fix feed

If the user has Summary selected for their RSS settings, the plugin should not run,
since it works on the full content, not the excerpt.
added vertical limit to email image size

Not necessary, but disconcerting to see 0 for the image size in plugins/widgets which
show it. Set as double the width so it will presumably not be hit.
updated README