Interfaces for fog-db

There's more than one way to implement a database for fog-db, and this crate is dedicated to creating a common API for dealing with fog-db databases. It defines:

• The cursor API for simultaneous, streaming navigation through both local and remote databases.
• The transaction API for modifying the local database
• The certificate and policy API for setting access policies into the database
• The Group API for opening up a group of connections to remote database nodes