New Pylearn2 wrappers and an example

Current issues:

* the example works 1.5 slower than the Groundhog one, and monitoring can not be blamed for that (tried to switch it off)

* binding of `data` and the cost computation graph inputs in `Pylearn2Cost.expr` should be done automatically

* a learning rule for embedding monitoring into the gradient computation should be written

@lamblin, you might want to take a look.