introduce EmptyExpression instead of null for [,,]

[Issue 525](https://code.google.com/p/esprima/issues/detail?id=525)

required for mdevils/node-jscs/issues/315
null don't have any information about where the token was.
for example:
```js
var x = [
  , 4,
  ,
];
```

WARN: I'm not sure about `lookahead`, is it right to use it there in parseEmptyExpression?