- formatters (both field and view) return elements formatted D7 style

- formatters are passing raw values to view and then to preprocess functions
- moved core formatters into separate file views_rss_core.field.inc
- added new field handler/RSS field formatter for "All taxonomy terms"
- added new generic field handler working with raw values received from formatters
- views_rss_core_preprocess_category() no longer used for feed items
- renamed views_rss_core_preprocess_category() to views_rss_core_preprocess_channel_category()
- views-view-views-rss-field.tpl.php modified to handle $markup variable
- updated view field settings forms
- updated function docs