Convey snapshot information

This partially addresses #46 - but does not completely resolve it.

Added here is a new `flocker-volume snapshots` command and a new method on `IRemoteVolumeManager` to invoke it.  This allows the pushing side of an interaction to learn what snapshots exist on the receiving side of that interaction.  A follow-up branch will use this information to generate incremental streams (using the functionality implemented in #657).

This branch is based on #657.  Review that first, then recompute the diff to review this one.