Allow attribute key to be set in Mapped Convention

The @ symbol is a reserved character in Internet Explorer, and prevents access to JSON objects in the canonical way (i.e. object.property). I am aware of the feature to suppress the @ symbol on output, but desire a way to pass back the same JSON objects from readers to writers.

The included patch adds the ability to change the key prepended to attributes to a character other than @. The feature is implemented in the same style as MappedXMLStreamWriter.valueKey and is controlled via the Configuration object.