GanttCalendarPlugin: follow-up to r852, fixed missing `pkg_resources`

git-svn-id: http://svn.sourceforge.jp/svnroot/shibuya-trac/plugins/ganttcalendarplugin/trunk@854 456a77e7-5939-0410-b211-84f85430c667
GanttCalendarPlugin: show datepicker for text field of holiday if Trac 1.0 or later

git-svn-id: http://svn.sourceforge.jp/svnroot/shibuya-trac/plugins/ganttcalendarplugin/trunk@855 456a77e7-5939-0410-b211-84f85430c667