[WIP] extracting learning rates to learning_rates.pxd and learning_rates.pyx

This pull request is for #3647
~~TODO: put adagrad and adadelta array inits in constructor~~
~~TODO: make adagrad and adadelta arrays nparrays (not malloc)~~
TODO: demonstrate adagrad and adadelta on large dataset (rcv1)
Ok, I've just updated this to use a numpy array combined with a pointer to the data so that should be all set!