Support unicode variable names in scope

[ES 5.1 allows unicode characters for identifier names](http://www.ecma-international.org/ecma-262/5.1/#sec-7.6) but angular doesn't. For example, the following code won't work:

```
$scope.がな = 1;  // fine
$scope.$watch('がな', function (value) {
    alert('new value: ' + value);
});  // doesn't work (Lexer Error)
```

How about supporting them in angular as well?
