XJSNamespacedName to replace namespace in XJSIdentifier token

Follow-up to https://github.com/facebook/esprima/pull/7

As usual, just shout if there's anything you disagree with or want done differently.

I'm kind of inclined to change `XJSIdentifier.name` to `XJSIdentifier.value` to signify that it's actually a value now, `name` kind of suggests that it is an object. Also, using `XJSIdentifier.value` would make it distinct from `XJSNamespacedName.name`. But it's a rather disruptive change.