Fixed handling of file names starting with a '/'

The API documentation for `HtmlHelper::css()` says:

>If $path is prefixed with '/', the path will be relative to the webroot of your application.

This also applies to `HtmlHelper::script()`. Now if I have a directory `subdir` containing scripts and/or stylesheets in my `webroot`, I might want to do:

```php
$this->Minify->css('/subdir/mystyle.css');
$this->Minify->script('/subdir/myscript.js');
```
But this would only work with `MinifyAsset` being disabled. Otherwise, the Minify library responds with a `400 Bad Request` response code.

In this patch, I've added some code to handle this: `$pluginPath` and `$type` are only prepended when the filename does *not* start with a '/'.