Compress map JSON for transfer to client

[pkraker]

The map JSON is currently transferred to the client uncompressed. As the JSON can get quite large, it would improve loading speeds to transfer it as gzip or a similar compressed format.

More details:
The service getLatestRevision.php pulls the JSON from the database. This service is called by the client with d3.json in mediator.js, which is where it's transferred to the client.