BSD dual licensing Bio/motifs/thresholds.py aka Bio/Motif/Thresholds.py

Cross reference #898.

File ``Bio/motifs/thresholds.py`` was originally checked in as ``Bio/Motif/Thresholds.py`` by Bartek Wilczynski (@barwil) and was credited copyright 2008 by Norbert Dojer:

https://github.com/biopython/biopython/commit/36bf6ced5ed649ef3435af8c1331c6892b972b2b

We will need Norbert's agreement to dual license this file - based on the commit authorship I believe everyone else who has contributed to this file has agreed.

Bartek, could you ask on the project's behalf please - agreement on GitHub is fine, or an email we can forward to our mailing list as a public record. Thank you!