Support TopoJSON as an input format

[robhawkes]

It's important that TopoJSON is supported as a valid input format. Some questions that need to be answered before we can do that:

• Should this be part of the same system that handles GeoJSON input?
• How should TopoJSON input be represented within the Blueprint API (eg. raw TopoJSON or GeoJSON)?

At first glance I feel that TopoJSON needs to be converted into some other format for easy, consistent use within ViziCities; with GeoJSON feeling like a good resulting format.