Multiple updates to resource_feeder, particularily RSS 2.0 feeds.
* added docs to rss.rb explaining default options and what each option does
* added support for optionally providing a pubDate for the feed (right pubDate is only at the item level)
* added better error handling if the user just keeps the default options, but their resource doesn't respond to any of the default method calls. Previously this would raise some sort of obscure type error - now it will raise an ArgumentError with an error message telling the user they probably have to override defaults.
If you would like more tests or better wording the docs, just holler.
* includes tests for the new feed level pubDate and the new error handling
* cleaned up a test duplication a little
* this patch also includes the fixes from Ticket #6787 to be compatible with the latest SimplyHelpful plugin - so if this patch is applied #6787 can be closed.
