Support non-memmappable types

[mdboom]

Examples would be a packed bit format, or anything where the "view" is not exactly the same as the data.

We should try to do this right, if possible, and not repeat the mistakes of pyfits.

[embray]

I've actually been really wanting to write a bit-array dtype for Numpy. I think this would enormously useful and worth the effort, especially for masks and page table-like applications.