Change license to BSD

Ondrej Certik agreed to this in private email:

"""
Hi Dag,

Absolutely! Just take the MIT license out.

Happy New Year!

Ondrej

On Tue, Jan 1, 2013 at 9:23 AM, Dag Sverre Seljebotn
<d.s.seljebotn@astro.uio.no> wrote:
> Hi, can I get your permission to take your MIT-licensed sources for restrict
> and rework them and license them under BSD 3-clause (which we agreed with
> Chris to put Hashdist under...), without having to drag in the MIT license
> and make things complicated (you'll just be co-copyright-holder for the
> BSD-licensed work).
>
> Happy new year!
>
> Dag Sverre
"""