SQL: Allow database name and schema be specified for each model

In larger applications, data is often divided between schemas and databases.

This change allows developers to define schema and database for each model that uses Postgres or MySQL database adapters. (Schemas only supported on Postgres.)

Interface is simple -- when defining a model, just add "db" and/or "schema" values:

```
var User = function () {
  this.db = 'mydatabasename';
  this.schema = 'myschemaname';

  this.property( ...
```