fixed url to calendar-icon image
changed widget class to use a items array
added abspath check to improve security
added and implemented shortcode attribute "show_starttime"

This is the preparation for the similar widget option (see Issue #16).
implemented hidden shortcode attributes title_length and location_length

required for similar widget option described in #16
fixed tuncate length in event details if a shortcode was used there
#16 added widget options "title_length", "show_starttime", "location_length", "show_details", "details_length"
only set links in widget if all required informations is available
fixed deleting of categories
Fixed issue #12: Dates not correctly localized in new/edit event view.
fixed and enhanced minDate for end-datepicker if start-date is changed
preparation for version 0.4.1
Merge branch 'develop'