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.
Added very brief test for JSON output.

Note that this only tests one call, so it can't be considered
any sort of comprehensive test - we will need to add more.
Report output mime type from `output_formatter`.

This means we can report the correct type in the HTTP response
header. Note that, at the moment, it's hard-coded to add a UTF-8
charset specifier. This might need to go away in the future, but
is fine shared between JSON and XML, both of which are UTF-8
anyway.
Added missing `id` attribute for JSON elements.

This fixes issue #61. The JSON test passes now.
Fix preprocessor error in experimental API 0.7.

Originally, all preprocessor definitions were passed on the
compiler command line, but this was changed to using `config.h`
a while ago. Seems that `api07/map_handler.hpp` was forgotten in
those changes. This fixes issue #60.
Merge pull request #57 from pnorman/gsoc13

Create a new pgsnapshot backend