text: Title: Bug when using Lasagne `mask_input` parameter
  Body: When initializing layers, the `incoming` and `incomings` arguments are resolved when they happen to be strings. However, those are not the only ones that may reference other layers. The `mask_input` parameter from recurrent layers also references another layer. Therefore, `initialize_layers` should resolve that too, elso the string will simply be passed on, causing a Lasagne error.
  
  There may be other cases in Lasagne, I'm not sure.