Casa masks

I took a crack at implementing a way to create CASA masks (partially addresses #9). 
There's a couple issues yet:
* I borrowed from [casa_tools](https://github.com/keflavich/casa_tools/blob/master/casa_tools/header_to_template.py) to write/read from temporary FITS files. This seems kinda clunky and some header info is likely lost in translation.
* Appending the created mask to an image isn't working ([see here](http://casaguides.nrao.edu/index.php?title=Masking_images_for_analysis)). Does anyone know of a better way to do this?
We should also have a function to turn a `SpectralCube` mask into a mask directly, though that won't be defined for all mask types.  For boolean masks, though, I think we want a `to_casamask` transformer.  This is mostly a note-to-self to raise a different issue, I think...