WIP: Vault End-Point Get/Head Update

DONE:
- Vault Head returns whether or not a Vault Exists
- Vault Get returns statistics on the vault
 - statistics are gathered from the drivers

Note: There are still several coverage issues in the drivers regarding how default values are returned.
deuce.drivers.cassandra.cassandrametadatadriver - lines 198-199
deuce.drivers.sqlite.sqlitemetadatadriver - lines 260-264
deuce.drivers.mongodb.mongodbmetadatadriver - lines 64,73, 82
deuce.drivers.swift.swiftstoragedriver - lines 86-87

Any suggestions on a good resolution welcome.

Note: Coverage doesn't like this. See my conversation with @TheSriram for the sqlite driver lines 260-264 regarding this.