Clean-up.
path package doesn't exist anymore.
Jackson is now embedded so no need to import its packages anymore.
Accessing mixin via getMixins requires the mixin's name to be escaped since that's how mixins are stored.
Use isNodeType instead of iterating twice over mixin node types.
Added reference to property representation to easily check whether the property is a reference to a node.
Added getByPathURI method to easily create a path URI.
If the property is of type Path, target should use a by-path URI.
Avoid creating parent JSONMixins to create a JSONMixin.
Implemented children initialization from JSON.
Added workspace name and language to URIs so that people can choose which data store to access.