Disambiguate licencing

[nckx]

Consider this a bug report with a patch attached :-)

sshuttle's current licence terms are a bit unclear. The LICENSE file is a copy of the LGPL, but none of the source files contain the suggested — if not mandatory — notice header.

I've added some, and corrected setup.py to no longer incorrectly declare the project GPL in places like PyPI.

I wasn't sure whom to credit. I'll gladly update the PR with any suggestions.