added rss feed option (not implemented yet)

issue #41
whitespace cleanup
make construct function private
Change RSS feed option caption

issue #41
added initial feed support

issue #41
added feed link in event list

issue #41
created seperate tab for feed options

issue #41
fixed cat_filter description
added widget option cat_filter

issue #43
added shortcode attribute "add_feed_link"; many feed code changes

issue #41
added escaping of attributes in shortcode output
changed feed itemdata

issue #41
added and implemented option "el_add_feed_link"

issue #41
removed unused part
added rss image to feed link

issue #41
renamed shortcode attribute "head_feed_link"

issue #41
added and implemented option "el_feed_link_pos"

issue #41
added and implemented option "el_feed_link_align"

issue #41
added option 'el_feed_link_text'

issue #41
added and implemented option "el_feed_link_img"

issue #41