16-bit monochrome support for JPEG2000

Rebase the monochrome 16-bit support on the current master. I screwed up the last fast forward, but this is all my changes. It seems to work according to openjpeg and my own tests. YMMV.

I have some smoke tests but they require either ``numpy`` or the ``openjpeg`` command line tools.  See https://gist.github.com/videan42/48d0542312c6b225d7eb. Should I add these as well or do you just want test images?

Closes #625