Added support for variable substitution when doing manual property value transformation (i.e. for instance using `-[InterfaCSS transformedValueOfStyleSheetVariableWithName:asPropertyType:]` or `-[ISSStyleSheetParser transformValue:asPropertyType:]`).
The full interface of ISSPropertyDefinition is now exposed in ISSPropertyDefinition.h (removed ISSPropertyDefinition+Private.h). Added method to register a ISSPropertyDefinition directly to ISSPropertyRegistry.