Use the request object in the FrontendURL.

This removes some custom URL parsing functionality that's allready done
in the Symfony request object.
Remove if statement that's allready tested.
Don't use $_GET if possible.

A new hasParameter method helps us use the URL object instead off the
$_GET array.
Use the Symfony Request object in the Backend URL.

This commit removes some functionality that already exists in the
Symfony Request object.
Remove unused private method.
Deprecate $URL->getHost in favour of $request->getHttpHost();
Merge pull request #957 from forkcms/request-in-url

Refactor URL class to use the Symfony Request better.