Thin pool improvements

Rebased ontop of @rhvgoyal's changes.  This pull request provides:
1) small udev scanning fix for the case where docker creates the thin-pool
2) new --storage-opt dm.thinpooldev to allow an admin to create/manage the thin-pool that docker uses outside of docker.
This else { was needed, otherwise dataUsed and transactionId weren't defined.
I'm new to go, so I'll defer to anyone with suggestions that are known to actually work.