Supporting String-Files

Used to change pypng-behavior on working with "files", actually with string-files: StringIO.

The IF-statement worked fine for _guess'es of file-instances, but what about StringIO-objects/instances which works just the same way ? They were ignored. 

Decided to test the class-name 'StringIO.StringIO', because there were no need for a real import of StringIO-lib. We don't need to use, we just want to test. The behavior is the same like files do.

I use pypnd on my fun-project... https://github.com/dennishafemann/text-source-picture