Change type of reserved words in the object

Per EcmaScript 5.1 spec reserved words are allowed to be keys in object, so it is more correct to represent them as "Identifier" type in token array.

https://code.google.com/p/esprima/issues/detail?id=481

I understand that change of arguments for `check-coverage` command is undesirable, or maybe even the whole approach is too. So if there is a better way to do it, please let me know.