Add hstore array support

This PR adds support for *hstore* array fields in Postgres.

The model constructor was also refactored a bit to normalize the *DataTypes* definition throughout the library. The separation of concerns between dialects still needs a bit of work but that can be fixed in the future. 

We've also [submitted](https://github.com/brianc/node-pg-types/pull/5) a PR to [node-pg-types](https://github.com/brianc/node-pg-types) that adds support for parsing `HSTORE[]` fields as an array of strings. In the meantime, we've updated the *pg* dev dependency to use a forked *pg.js* so that travis can build and test this PR.