Plugins for Nuke (Foundry product) for reading/writing OIIO textures.

* txReader will read OIIO texture files (TIFF and OpenEXR, tiled, mipmapped)
  as Nuke input images.

* txWriter will write image files from Nuke as proper OIIO texture files
  (tiled, mip-mapped, all metadata set correctly).
Improve oiiotool help message
oiiotool -stats on deep files now prints a histogram of samples/pixel.

Also, for deep images that contain no depth file, skip erroneous printing
of min and maximum depth.