Upgrade from v2 to v3

The biggest change with this release is that BitBadger.Npgsql.Documents became BitBadger.Documents, a set of libraries providing the same API over both PostgreSQL and SQLite (provided the underlying database supports it). Existing PostgreSQL users should have a smooth transition.

• Drop Npgsql from namespace (BitBadger.Npgsql.Documents becomes BitBadger.Documents)
• Add implementation (PostgreSQL namespace is BitBadger.Documents.Postgres, SQLite is BitBadger.Documents.Sqlite)
• Both C# and F# idiomatic functions will be visible when those namespaces are imported or opened
• There is a Field constructor for creating field conditions (though look at v4's changes here as well)