Add examples

[klartext]

Please add examples for the base use cases, for example reading and writing an image and analyzing image data.

[cfcs]

If you have some concrete ideas we could add those to an examples/ directory; I'd be in favor of that.

For now, see the newly merged code:

• imagetool reads and writes pictures, amongst other functionality.
• basic unit test shows how to create an image and convert it to a PNG file byte string (and reading it back)
• Image.compare shows how to deal with the pixel data.