Use arrays for center and bounds in tilejson

Fixes #4

This does not validate that the parameter in the Mapnik XML is valid,
but if it is valid, it will result in a valid TileJSON.

cc @zerebubuth 
I think this would be better if there were a `static const std::unordered_set<std::string> array_keys` declared above and the test were:

```c++
if (array_keys.count(row.first) > 0) { ... }
```