added license file

Copied from https://code.google.com/p/hoverzoom/source/browse/src/trunk/license.txt

Ralph: It's very important you have a license file for forked code, since you are not the copyright owner. I think you must keep Romain's original copyright notice and not change the licensing terms.

As for your own copyright, I think you should add your name to the license file since you have made non-trivial changes (ce15e11). No need to document which files you touched since this is in SCM history.