corrected description of filterbar_items attribute (item option names in examples)
fixed regex in truncate function

The old version didn't match the h1, h2, ... elements correctly and
caused a wrong </h> closing tag.
some whitespace changes; removed "visible" field, added "hidden" field only where required
added and implemented shortcode attribute "initial_event_id"
some shortcode atts descriptions and whitespace changes
fixed initial_event_id handling
added option to show upcoming events only in the feed

issue #64
preparation for version 0.6.5
small readme fix
Merge branch 'develop'