Added ability to set route prefixes

Added ability to set route prefixes so that Toro can easily be used to serve content from a subfolder.

Now we can pass the folder prefix as the second parameter to Toro::serve() and all paths will be correctly matched inside the subfolder.

Example (Serving application from /mysite)

```php
Toro::serve(
    array(
        "/" => "HelloWorld",
        "/about" => "About",
    ), '/mysite');
```
All routes will now have /mysite prepended to them, so in our example the routes would be:
/mysite/
/mysite/about