Switch to the 2-clause BSD license

The "no-endosement" clause is meaningless because libtrading is a
community project.

Acked-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Acked-by: Marat Stanichenko <mstanichenko@gmail.com>
Signed-off-by: Pekka Enberg <penberg@iki.fi>