Use model._mptt_meta rather than model.__meta.

django-mptt  >= 0.4.0 moved the mptt options from model._mptt_meta to
model.__meta.
feincms >= v1.8.0 started using django-mptt  >= 0.6.
Add CHANGELOG.
Unify CHANGELOGs.
Merge pull request #10 from incuna/mptt-updates

Use model._mptt_meta rather than model.__meta.