Bigendian Fixes

Fixes for images on bigendian machines. Fixes #385

* Image formats were incorrect for I;16 for converting to numpy. 
* That exposed issues where tiffs from libtiff were put in native byte order, and native tiffs were in image byte order.
* Save tests were failing due to metadata issues, which are also fixed. Probably fixes #387, needs tests for that.

Undone, and worth checking out in the future: I;32 and F;16/32 formats, as well as I;16S.