new m2m introspection code not necessarily the perfect solution

My biggest issue is using the before table modifier in ::ResultSourceProxy::Table to add the result class m2m metadata accessor: I'm sure there is a better place to add that. Otherwise all is fine though perhaps it would be useful to add even more metadata or maybe change the hash keys?
@gregapompe / @racke: Comments?
(see #36 for details of old m2m issue)
