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?
Since this is actually creating a file and writing to disk, it would be good to set `dtype='int16'` in the empty array; that will reduce the data size by a factor of 4 from the default 64-bit float.