Extract state/{storage,txn} as top-level projects

juju/juju/state/storage is now juju/blobstore
juju/juju/state/txn is now juju/txn
Fix import ordering
Merge pull request #296 from axw/extract-state_storage_txn

Extract state/{storage,txn} as top-level projects

juju/juju/state/storage is now juju/blobstore
juju/juju/state/txn is now juju/txn