Make the in-memory filesystem snapshot listing return a Deferred as it should have before.  Also fix the test to catch this.
Introduce ``snapshots`` on the volume manager interface.  Add a few tests that loosely constrain its behavior.
Remove some redundancy from some existing volume manager tests.
Implement ``snapshots``.
Add a test for `flocker-volume snapshots`.
Implement `flocker-volume snapshots`.
Lint.
Merge remote-tracking branch 'origin/master' into use-snapshot-information-46
Merge pull request #658 from ClusterHQ/use-snapshot-information-46

Expand the remote volume manager interface so it can convey information about the snapshots that exist for the filesystems it manages.