This is needed for e.g. hooking the copr's webhook feature into Travis
build in github.  Use-case:

Some upstreams (notable example pgjdbc) want to have the copr build
(with build-time tests) done for each pull-request, and block the
PR unless the RPM build succeeds.  It is now non-trivial to get the
final build-status from copr, because the webhook call doesn't return
the build-id.

I'm not sure the actual webhooks have to be necessarily changed, and
return all the submitted build-ids (several builds might be triggered
by one webhook call);  we might as well implement bug 1344191 on
say /webhooks/package/<copr_id>/<package_name/<uuid>/ url -- and
add the allocated build-id into the response JSON.

As a followup feature, we could have:
$ copr-webhook --project <COPR_ID> \
               --package <PACKAGE_NAME> \
               --uuid-token <SECRET>
.. which would be able to parse the response.