Implement fallback for getting the size of a container

This moves Driver.Size() to Differ.DiffSize(), removing the empty
implementations in devmapper and dummy, and renaming the one in aufs.

Then we fall back to a container.Changes() implementation in the non-aufs
case.
Merge pull request #45 from alexlarsson/dm-plugin-diff-size

Implement fallback for getting the size of a container