ProjectionLayer fix parameters

[zseder]

When we use a bigger vocabulary than what is in training data (external embeddings at evaluation time for example), there are a lots of invariant parameters in ProjectionLayer.
We should create our own ProjectionLayer (inheriting the one from pylearn), that knows which parameters are constant, and which are changable, and then change ProjectionLayer.get_params() to return only that part of W that is changable (if this slicing operation is permitted in theano)

[zseder]

there is MLP.freeze(self, parameter_set) for this