celery: introduce make_app instead of creating app at import time

It is dirty to instantiate things at import time (unless it really is basic
singletons).

In 0.5.1 (and earlier), such dirtyness made partial test execution fail when
other things had global side effects and things didn't use the usual import
order:

$ py.test kallithea/lib/
collecting ...
――― kallithea/lib/celerypylons/__init__.py ―――
kallithea/lib/celerypylons/__init__.py:58: in <module>
app.config_from_object(celery_config(tg.config))
kallithea/lib/celerypylons/__init__.py:28: in celery_config
assert config['celery.imports'] == 'kallithea.lib.celerylib.tasks', 'Kallithea Celery configuration has not been loaded'
data/env/lib/python2.7/site-packages/tg/configuration/tgconfig.py:31: in __getitem__
return self.config_proxy.current_conf()[key]
E KeyError: 'celery.imports'

Avoid that by running a "factory" function when the celery app actually is
needed.