Previously, it was a publically accessible but invalid value (valueEmpty).
* Deprecate internal use of UndefinedValue(), NullValue(), FalseValue(), TrueValue()
* Guard against Empty, Result, Reference values from escaping the package
* The "reflect" package is willing to expose unexported fields in structs (a little suprising).
* Skip unexported fields when enumerating or trying to access (pretend they don't exist).
* Allow structs that implement json.Marshaler to work as expected in a JSON.stringify call.
If we let the runtime try to access unexported fields, it'll cause a panic.
This fixes#64
