* Added YamlPresenter*Error exceptions
* Added raises pragma to present()
* catch exceptions from YamlStream and Stream in present() and
put them as cause into YamlPresenter*Error
* Added loadToJson() proc
* Give "~" the type hint yTypeNull
* Always output "null" for yTagNull/yTypeNull scalars when
presenter is in JSON mode
* Added YamlWarningCallback that may be used to capture
parser warnings
* Removed yamlWarning as event kind
* Replaced checks for well-formedness in presenter with asserts
* Added checks for JSON compatibility of YamlStream in presenter
* Added proper type hints for special float values in
serializer to be able to check for them in the presenter
* Yield tag of custom type serialization when using ytsRootOnly,
but don't yield tags of map keys or values
* Yield proper tag of Table[K, V] serialization if requested
* renamed dumper.nim to presenter.nim to conform to YAML spec
Felix Krause