WIP: Masked arrays

Before I start implementing all the operations for masked arrays, it'd be great if someone (e.g. @nouiz) could let me know whether this is correct i.e. whether the files are in the correct place, whether I derived from the correct classes, etc. This is my first attempt at a PR to Theano, so I'm not sure how some things should be done.
`self.ttype` prints as `TensorType`, so `Masked{}` actually prints as `MaskedTensorType`. I'll change it so that it is clearer in the code.