celery: signal handling fix

* Fixes issue where signals handler could be called multiple times
  when multiple celery applications where created.

* Moves test case to celery package.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
celery: test case helper

* Adds testcase class helper for easier testing of Celery tasks. The
  helper will create a new Celery application per test, similar to the
  way InvenioTestCase creates a Flask application per test.

* Adds example test case using the new helper.

* Removes no longer needed test runner hack.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
installation: PyLD 0.5.0 upgrade

* Upgrades PyLD to version 0.5.0 to fix Travis builds on
  Python 2.6.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>