Allow to defined layout by param

This changes allows to define a layout for a specific page inside routes.
If no layout defined it use default (HighVoltage.layout)

f.e.

    get 'pages/home'   => 'high_voltage/pages#show', :id => 'home'
    get 'pages/mobile' => 'high_voltage/pages#show', :id => 'mobile', :layout => 'mobile'