Initial work on moving aufs to a subpackage
Rename graph backends to 'drivers' which is probably more self-explanatory
Merge pull request #2541 from shykes/graph-backend-rename

Rename graph backends to 'drivers' which is probably more self-explanatory
Integrate devmapper and aufs into the common "graphdriver" framework.

aufs is still enabled by default, no mechanism for switching drivers
yet.