Create a new pgsnapshot backend

This backend is intended for a pgsnapshot database loaded with osmosis.

It supports all non-history calls and passes all openstreetmap-api-testsuite
tests for non-history calls. The most notable history call is the 404 vs 410
error code of single-element fetch calls.

There are not yet unit tests for the backend.
Merge pull request #57 from pnorman/gsoc13

Create a new pgsnapshot backend