Postgres-specific functions - schema management

Relevant to #284.

This code introduces a way for each dialect to have it's own query, schema, and migrate methods by implementing a `*Interface` on the dialect's `Client_*` object. For the postgres schema functions I followed the convention of having a methods.js file, but this approach doesn't require that.

Beyond that, it implements functions for creating/dropping postgres schemas and reading/setting the search path.

Happy to make any adjustments or hear any other ideas/critiques.