Attempt to add RSS functionality

Currently this does not work, but will render an empty page with XML
header. This has something to do with Bubble/Rss/etc/config.xml being
setup incorrectly because currently the
Bubble/Block/Catalog/Stockmovements.php does not execute at all.
Further research is needed.

P.S. Also, if you are using the OpenERP Magento Connector this will
prevent stock from being exported (the event observer in magento
catches it and clears the cache for the block Stockmovement).