[Bf-taskforce25] DerivedMesh stuff

[joe]

I've decided to make some modifications to DerivedMesh, to handle
ngons in the bmesh branch.  Basically, I'm going to rename the
existing face accessors (to make it clear they access tesselations
now, not real faces) and add in some form of face iterators.

Brecht: the code you did on this a while ago would be nice to have, as
would any comments you have.

Joe