Add documentation for `RKCompoundValueTransformer`
Add header import to suppress warnings about search indexer
Add an optional `name` property for `RKValueTransformer` and implement `description` for `RKValueTransformer` and `RKCompoundValueTransformer`
Add support for subscripted index access to `RKCompoundValueTransformer`
Remove a transformer when `insertValueTransformer:atIndex:` is called to ensure it only appears once within a given `RKCompoundValueTransformer`