#522 Switch to BSD license

I think everyone has agreed to switch the license for GalSim from GPL to BSD.  This PR is mostly about the text of the new license.  I didn't update all the file headers yet.  I'll do that on master once this is merged in.  But please look at the proposed text and see if anything ought to be amended.

I went with the so-called ["2-clause license"](http://en.wikipedia.org/wiki/BSD_license#2-clause_license_.28.22Simplified_BSD_License.22_or_.22FreeBSD_License.22.29), or FreeBSD license.  It seemed like we probably don't care about the endorsement clause.  Please speak up if you disagree.

There is one sticking point -- the hsm module is apparently licensed under GPL.  So we would need to get that switched if we want to include it in a BSD-licensed GalSim.  Rachel, is this a problem?  I guess you probably need to clear it with Chris (and Uros?).