CLN: make license file machine readable
Splits extra information about the license and copyright holders to
AUTHORS.md.

The [gem used by GitHub's license detection](https://help.github.com/articles/licensing-a-repository/#detecting-a-license) now properly detects the correct license.

```
$ licensee
License file: LICENSE
License hash: d6dd3ea94544765085d28014f2ab3628706b9017
Attribution: Copyright (c) 2008-2012, AQR Capital Management, LLC, Lambda Foundry, Inc. and PyData Development Team
License: BSD 3-clause "New" or "Revised" License
Confidence: 100.00%
Method: Licensee::Matchers::Exact
```
