Add ability to return ZY images

[DocSavage]

Currently, a YZ slice returns Y in the X axis and Z in the Y. Allow ordering of the axis for 2d image GET as well as image tiles. This came up during CATMAID integration during Hackathon.

[unidesigner]

Any chance that flipping the image could be added as an option to DVID?

The Catmaid doc notes the issue on http://catmaid.readthedocs.io/en/stable/tile_sources.html#dvid-imageblk-voxels with "Because DVID prefers YZ axis ordering over ZY, note that tiles for that orientation must be transposed to be consistent with other tile source types." Not sure where that transposition should happen. Thanks for considering!