pg-based db module

This addresses #15, with the following new features:
* Improved/fixed transaction support
* Connection pooling (automatic, thanks to pg)
* LISTEN/NOTIFY support through EventEmitter-style .on(), on a connection
* Optional connection reuse
* Incremental query results (using Q#progress())
* Very large query support (using {bufferRows: false} with queries)

This removes a very large chunk of our Sequelize dependency. The only thing left
now is the migration script, which is still very useful.