Fixed bug in AstNodeImpl.setProperty(): Property was not set
SimpleTypeNameMapper can be initialized with another SimpleTypeNameMapper
Extracted superclass AstConverterBase from AstNodeConverterBase to facilitate non-Node JSON converters
AstNodeConverterBase now uses a cache to speed-up getter lookups
Changed semantics of storeTypes to alwaysStoreType.

Preparation for implementation of ptk-json-tools:
- Changed semantics of some methods in AstNodeConverterBase base class
- Renamed AstNodeConverter* to AstNodeXmlConverter*
Rewrote ptk-json-tools
Cleaned up TestAstBuilder and split it into individual java files for nodes and the factory.