general function for date format and daterange format handling

* moved available formats to options class
* added date formats for month and day
removed debug messages
adjusted date_formats and daterange_formats for using it also in admin about page
small css fixes in admin about page
added date and daterange format help on admin about page
small changes in admin about page
issue #65: adapted editor settings for event details similar to posts settings

consider wpautop also in event details output
updated date_filter description

issue #70
fixed get_date_range and past function
added "Past" in admin event list date filter selection
preparation for version 0.6.7
Merge branch 'develop'