Python3 Compatibility

Hello,

I integrated your excellent python-wavefile module into my Python3-based project, but I had to make some changes to use it with Python3, so I am offering the changes back to you for inclusion in your project.

As far as I know, the changes did not break Python2 compatibility, but I would encourage you to test it before merging.

The only potential regression I can think of would be in commit 21f291e, I'm not sure if using the ASCII encoding is valid for systems not using en_US.

Thanks,
Jeff