changed license to BSD 3-clause

- fix #9
- The code is currently under GPL because MDAnalysis is under GPL.
  However, we want to keep MDAnalysisData general and don't want to import
  MDAnalysis so we can keep the license less restrictive. Most of the code
  originally came from sklearn so we should just go back to the sklearn
  license, which is BSD-3.
- At the moment the only code contributor is @orbeckst and he agrees to this change.

Author: orbeckst