- changed way how feed is built: use format_xml_elements() instead of theming

- updated all preprocess functions to be compatible with format_xml_elements() element format
- removed function views_rss_htmlspecialchars() and all calls to it - this is not done automatically by format_xml_elements()
- updated views_rss_rewrite_relative_paths() to handle multiple elements and arrays of subelements
- updated element definitions to account for updated/removed preprocess functions
- added hook_views_rss_feed_alter()
- element values are no longer going through any theme() function
- removed theme callbacks: views_view_views_rss_fields and views_view_views_rss_field
- removed template files: views-view-views-rss-fields.tpl.php and views-view-views-rss-field.tpl.php