Rename Snapshot -> Snapshots.

Main reason is to free up noun to allow core data types to be consistently
named. This is the first step in my plan changes, PR'ing so that
it can be cherry picked by others if required given it is a bit
of a pevasive little thing.
Merge pull request #423 from ambiata/topic/snapshot-rename

Rename Snapshot -> Snapshots.
Some renames for consistency and to make sense.