Fix Addressbook::childExists($id).

Return false instead of throwing an exception if there's no child with $id.
Also, add a test for that function.
Fix log output in purgeIndexes().
Add displayName() to GroupController.
Prepare for sharing of contact groups.
Delete groups by their id instead of their name.
Rename group by id.
Fix and re-activate Contact sharing backend.

While not needed for actual contacts sharing (which is solely done via
the Addressbook sharing backend), we need its isValidSource() function
in core's \OC\Tags::getIdsForTag() (which implements \OCP\ITags::getIdsForTag()).
It is required there to test if the items that are tagged with a given
tag are actually allowed to be accessed by the user, i.e. owned by the
user or someone who is sharing it with the user.
Include shared groups.

Fixes #38.
Replace "if (is_null($a) || $a === '')" clauses by "if (empty($a))".
Merge pull request #668 from ockham/share-contact-groups2

Share contact groups