Merge pull request #262 from bintoro/modelmeta-deepcopy

Make copies of inherited meta attributes when setting up a Model
Merge branch 'compound-allow-none'
Support for polymorphic model fields

* New type PolyModelType supports import conversion into one of
a number of model classes based on what the data looks like.

Accompanying features:

* Field objects have a new attribute 'name' reflecting the name
under which they appear within their models.

* Each model class now contains a registry of its subclasses,
accessible via the attribute `_subclasses`.
Added 'allow_subclasses' keyword option
PolyModelType final commit
Merge branch 'polymorphic-model-type' into future

Conflicts:
	schematics/models.py
	schematics/types/compound.py
	tests/test_polymodeltype.py