Version 2.2.0

Changed

• NODATA and TRANSPARENT values are merged. [#41]
  • User visible change: paletted PNG outupt images will have up to 254 color
    values and a 255th value that is transparent.
  • Internal code changes: removes TRANSPARENT_IDX (254) and uses
    NODATA_IDX (255) in its stead. A color of (0,0,0,0) was previosly set to
    both the indexes (254 and 255) in the ouput PNGs and now only 255 will have
    this value. This change ensures the roundtrip from single band to RGBA to
    paletted PNG is consistent.