Fix licensing issues

I want to package your python library in Debian to make it available for a program I use. In order to do that, I need to have a release that fixes the licensing issues that your source tarball contains:
- sourceless minified js : Even better would be to remove the minified code from the source.
- add full MIT license header as required by license in txsockjs/websockets.py

I have prepared a pull request fixing those. It would unblock my work if you could do a release taking into account those changes.