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.
Updated section for 0.9.12.
Added support for specifying the collection view layout class (via the `layoutClass` attribute) when creating collection views in a view definition file.
Added method `elementNameToViewClass` to `ISSViewHierarchyParser`, to make it possible for subclasses to override mapping of tag name to view class.
Added section for version 0.9.13.
Updated for changes in 0.9.12.