Parse NVG XML to NVG JSON

[spatialillusions]

Add functionality to parse NVG XML to NVG JSON.
Should support:

• NVG 1.5 XML
• NVG 2.0 XML

[spatialillusions]

Now supports evertygning except metadata and extensionbecause of lacking schema support.

[spatialillusions]

metadata is now implemented and a general implementation of extension is the only thing left.