Support the most common IO formats

[Robzz]

It would be good to support the most common image types from the most popular image formats. Let's start with

• PNG
  • RGB
  • Grayscale
  • RGBA
  • 8bit
  • 16bit
• GIF
  • The gif crate looks pretty good
• JPEG
  • No encoder available (but maybe we can put the jpeg encoder from image in a separate crate ?)
• TIFF
  • Encoding is not supported by the tiff crate yet
  • Neither are greyscale images (rgb too?) with alpha channels apparently
• BMP
  • No usable crates for us apparently. Maybe we can extract the code from image ?