added snapshots and bind mounts

with bind mounts you can share a directory between the host and the
container. with the snapshot you can initialize a container with a copy
of some data.
added snapshot command and some fixes

Snapshot command to create, delete and list snapshots. It's also
possible to add multiple bind or snapshot parameters to the run command
now.
moved buildstep output before the layer check
removed debug logging and fixed buildstep cleanup
recursively create snapshots
Merge pull request #3 from teemow/snapshots

added snapshots and bind mounts