Writing PNG with pure Python

[Johann C. Rocholl]

> You should really also include the alpha channel. Without that, PNG is
> crippled IMHO.

I have now added simple transparency (marking one color as transparent
with a tRNS chunk). If anybody wants full alpha channel support, ask
kindly or send me a patch. I would like to avoid duplicating all the
functions, so maybe we should introduce a parameter to switch between 3
and 4 bytes per pixel.

Cheers,
Johann