Don't allow to set service retirement date to the past

This fix initializes the dhtmlXCalendarObject when setting
service retirement date in such a way that it would not
render past days as selectable.

This effectively disallows to select a date in the past
for service retirement.

https://bugzilla.redhat.com/show_bug.cgi?id=1124555
Merge pull request #392 from mzazrivec/fix_service_retirement_date

Don't allow to set service retirement date to the past