Move aufs to a storage driver, add devicemapper and dummy drivers

This pull request moves storage capabilities to its own subsystem, and adds a driver API for pluggable storage backends. The driver system is used to store containers, images and volumes.

[X] Implement driver API
[X] Move aufs to its own driver
[X] Integrate lvm/devicemapper implementation by @alexlarsson into a driver
[X] Implement a new "dummy" driver which uses regular copy instead of copy-on-write
[ ] Implement compatibility layer with existing containers, images and volumes
[ ] Full test coverage on all drivers