added model-relationship-introspection helper
added model-relationship-introspection helper

okay, this is cool, but a little complicated.

one of the awkward things about allowing multiple apps to OPTIONALLY
interact is that it requires additional code on both ends. for example,
if we want to optionally add a "location" field to a Reporter when the
"locations" app is running, we need to perform that check all over the
place (the model(s), add/edit views, and templates all need to include
the new field) -- and that's for a single coupling.

this commit is my attempt to start automating the integration. as an
example here, the "locations" app, which is completely unaware of the
"questions" app, lists the number of q&a submissions linked to each
location automatically. (also, the number of reporters linked to each
location -- but the app depends upon "reporters", so that's not very
surprising).

peterayeni/rapidsms,unicefuganda/edtrac,peterayeni/rapidsms,ehealthafrica-ci/rapidsms,peterayeni/rapidsms,unicefuganda/edtrac,caktus/rapidsms,lsgunth/rapidsms,lsgunth/rapidsms,rapidsms/rapidsms-core-dev,rapidsms/rapidsms-core-dev,eHealthAfrica/rapidsms,catalpainternational/rapidsms,lsgunth/rapidsms,lsgunth/rapidsms,ehealthafrica-ci/rapidsms,ken-muturi/rapidsms,caktus/rapidsms,catalpainternational/rapidsms,dimagi/rapidsms-core-dev,dimagi/rapidsms,ehealthafrica-ci/rapidsms,catalpainternational/rapidsms,ken-muturi/rapidsms,dimagi/rapidsms,caktus/rapidsms,catalpainternational/rapidsms,peterayeni/rapidsms,dimagi/rapidsms-core-dev,unicefuganda/edtrac,eHealthAfrica/rapidsms,eHealthAfrica/rapidsms,ken-muturi/rapidsms