luci-app-ddns: Update to support ddns-scripts 2.1.0-1

fix verify of entry for DNS server Issue #244
	https://github.com/openwrt/luci/issues/244
add support for option 'update_script'
add display of version information when click on "Dynamic DNS" on
overview page
add verify of installed ddns-scripts version and show as hint if not
correct version
modified epoch to date conversation
cbi object Flag did not set section.changed state, fixed in
tools.flag_parse function
ucitrack entry no longer needed and removed
minor fixes

Signed-off-by: Christian Schoenebeck <christian.schoenebeck@gmail.com>