Interface changes in genseq

Changed interface of SequenceGenerator and Emitter for beam search.
OK, I see. Than I guess it should accept `output` as inputs, not `readouts`.

Just to clarify what the difference is: readouts are the last thing you get with neural network (typically pre-softmax energies) from which the `Emitter` can sample outputs with its `emit` method. Outputs are the elements of the target sequence, i.e. indices of phonemes or words.