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)~~ - my iterator is slow and Groundhog does not take this time into account

* ~~binding of `data` and the cost computation graph inputs in `Pylearn2Cost.expr` should be done automatically~~ - done, the names of the input variables are matched with sources names from `Dataset.data_specs`

* ~~a learning rule for embedding monitoring into the gradient computation should be written~~ - done, together with an extension to take averages

* ~~there not always is the top brick, which is assumed by the current Model wrapper~~ - can wait 

* ~~long-term objective: the bricks and the parameters should be extracted from the cost computation graph. Would require going into scan, and thus should not expected soon~~ - we will not recurse into scan any soon

@lamblin, you might want to take a look.