src block deletion patch

Fixed:
in Source configuration page,
* "Remove all" icon button didn't remove empty src block from
squidGuard.conf.
* Now removes the src block if the element in quetion was the last one
of the source group.
* Now the last element of a source group still in use denies deletion.
Merge pull request #26 from Tatsuya-Nonogaki/master

src block deletion patch