Change file license headers to match LICENSE file and setup.py (MIT License).

This resolves #3 (closed) by changing the inconsistent file headers from GPL to MIT licensing. The issue conversation on #3 (closed) indicates approval from all major contributors to the code (@erhart, @erikfransson, @mattias.slabanja). I looked at all the files with GPL headers and all the commits to those files are by one of the aforementioned people, with one exception: @evoyiatzis made one commit (e988c233) when the file header was GPL and the LICENSE file was MIT. I am not a lawyer, but the maintainers might wish to seek approval of all those tagged on this PR.