Re-license as 2 BSD

Hi @pooler! I'd like to work with your cpuminer software, but before I make any code changes, I wonder if you'd be receptive to re-licensing under the 2-clause BSD license, found here:

http://choosealicense.com/licenses/bsd/

I personally do all my usual software work under either the BSD or MIT licenses, since I find that such projects are more amenable to pick up from for-profit, government, and non-profit organizations alike, and don't depend on the extremely long and difficult to read GPL license.

I understand this might be a touchy subject, and perhaps you chose GPL for this project for reasons beyond just being a common default choice for open source software -- for example, perhaps you do intend to compel modifiers and users of your software to provide all fixes -- even silly ones -- back to the open source space.

People have opinions on licensing (and I'm no exception). If you don't have inflexible opinions, I'd be happy to discuss this with you either in this PR, or in private e-mail, or whatever. In the meantime, if you don't have a strong opinion already, you might take a look at:

http://choosealicense.com/

Please take a moment to consider this change, though. I feel like your project could benefit from a wider audience than GPL tends to allow, especially since your project has such direct for-profit uses.

Thanks!