Read "propel_converter" options from request attributes

[gharlan]

closes #448

[gharlan]

ping @havvg and @marcj

On many pages in my app it brings a performance boost of ~40%.

[gharlan]

On many pages

Pages where I'm using implicit ParamConverters like this:

    /**
     * @Route("/foo/{id}", name="foo.view")
     */
    public function fooAction(Request $request, Foo $foo)
    {
    }

[gharlan]

ping :)

[gharlan]

ping ;)

[marcj]

Sorry, @havvg is away somehow. Merged, thanks :)