missing bitwise xor function

Fixes error:
```
[object Object] has no method 'Node_Expr_BitwiseXor' 
```

Which results from using xor e.g:
```
error_reporting(E_ALL ^ E_NOTICE);

also reported to phpjs project:
https://github.com/niklasvh/php.js/pull/48
Merge pull request #1868 from adamjimenez/patch-24

missing bitwise xor function