Move (still hard-coded) DB connection out of Repository.

* DRY it up.
* Rename it to data_store.
  * It's actually a DB connection at the moment.
  * Might rename it or refactor it later.
* Move it out of Repository.
* Move it to Preserves.
* Use it in RSpec helper to reset DB.
Stop hard-coding the DB connection.

* Add `Preserves.data_store=` to set it.
* Change `Preserves.data_store` to use the value set by `data_store=`.
* Add `Preserves.PostreSQL` to create a connection to use as a data store.
* Set the data store in the spec helper.
Mapper: pass Repository into initializer.

Not sure if we'll need this long-term, but will need it short-term
to implement a simple (naive) `belongs_to`.
Add belongs_to relations.
Add TODO file.

I've had this file around for a while. Just committing it now.