IE8 error when entering path with "enter" functions registered.

Path.js version 0.8.4

Error: Function.prototype.apply: argument is null or undefined 

Possible fix: Line 180, remove null parameter:

```
result = Path.routes.defined[this.path].do_enter[i].apply(this);
```

After change works in IE8, IE9, Chrome 20, FF13 on Windows.
