fixed wrong url for single event page link
small change for background-color in date
small changes in date selection
add id to url-get-parameters to fix issue with different shortcodes on one page

This is especially required for the widget on the event-list page.
fixed page and links when attribute initial_date is set
added widget option "show_location"; changed widget layout
changed required prevelegs for admin about page
add visible field to atts array; added and implemented attributes "url_to_page" and "sc_id_for_url"

Both attributes are required for the widget.
included the required option to generate the shortcode "sc_id_for_url"; renamed option "link_to_page_url" to "url_to_page"
updated help messages
fixed url from widget to event details with permalink_structure
updated screenshots
preparation for version 0.3.1
Merge branch 'develop'