URLRewrite doesn't rewrite assets in the same directory

The sprite URL in the Chosen library was not being rewritten, for example:

```css
background: url('chosen-sprite.png') -42px 1px no-repeat;
```

Looks like it's because of this line in the URLRewrite filter:

```php
if (strpos($url, '/') != 0 && $this->fileExists($this->root . $url)) {
```

`strpos()` returns `false` in this case, so the whole if condition fails.

I think it was supposed to ignore URLs that start with a `/`, so I've changed it to just check the first character instead of searching the whole string.

(I also removed some unused variables - looks like they were copied from `relative_match()` further up.)