JSON data encoder wrapper supporting GeoJSON and JSON-FG

[navispatial]

Currently in geobase version 1.0.x there is a separate internal GeoJSON decoder implementation, but GeoJSON encoder is shared with WKT encoder. This means that the encoder is not very "JSON object tree or JSON text oriented", making it difficult to add capabilities like pretty printing (of JSON text output).

When #53 shall be implemented a new encoder for JSON based geospatial data formats could be designed:

• content writer for JSON object tree (wrapping standard JSON encoder)
• GeoJSON and JSON-FG geometry writers (implementing geometry content interface)
• designed so that manipulating JSON formatting is possible while streaming data (geometry objects in memory -> geometry content interface -> GeoJSON or JSON-FG encoder -> content writer for JSON object tree -> standard JSON encoder)