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.
I don't like the name MakeMask. For me it mean, make a mask, not make a masked array object. What about MakeMaskedTensor()? Also, use a singleton instance.