Composite layer with routing

This extends the MLP CompositeLayer class to perform routing of inputs to subsets of its components. It's based on Ian's code for DBMs (PR #636). See also Laurent's message on pylearn-dev: https://groups.google.com/d/msg/pylearn-dev/qgHkMS7cjP0/P7Xn_UqrLV0J

@axeldavy @laurent-dinh