Provide json dump importer

[hbruch]

Photon already supports the export of json dump files in elasticsearch's import format via the '-json' flag.

As discussed in #288, photon should provide an option to import such an json dump file.

This would have a couple of advantages, like

• speed up import times compared to the current nominatim import
• allow us to provide region dumps
• easier setup of photon, i.e. for new contributors
• the possibility to supply custom data as json file