Writing PNG with pure Python

[Johann C. Rocholl]

The MIT license is enticingly short and simple, thank you for the tip.

I have now decided to license my project (including the pure python PNG
library) under the Apache License 2.0 which is less restrictive than
the GPL in terms of sublicensing. The Apache License looks modern and
well-maintained to me. My project runs on Apache, so there is some
context there as well. Also, this change will simplify things if I ever
want to contribute some of the code to the Python Software Foundation.

Cheers,
Johann