CLN: make license file machine readable

[tswast]

Splits extra information about the license and copyright holders to
AUTHORS.md. Uses plain text license so that string matching of the
license file works better.

The gem used by GitHub's license detection now properly detects the correct license.

$ licensee LICENSE.txt
License file: LICENSE.txt
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

Fixes #30.

[codecov-io]

Codecov Report

Merging #56 into master will decrease coverage by 44.87%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master      #56       +/-   ##
===========================================
- Coverage   74.18%   29.31%   -44.88%     
===========================================
  Files           4        4               
  Lines        1511     1511               
===========================================
- Hits         1121      443      -678     
- Misses        390     1068      +678

Impacted Files	Coverage Δ
pandas_gbq/gbq.py	16.08% <0%> (-63.14%)	⬇️
pandas_gbq/tests/test_gbq.py	31.88% <0%> (-49.94%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cec8c86...38e1542. Read the comment docs.

[tswast]

I've filed a similar pull request on the Pandas repo as well: pandas-dev/pandas#16649

[jreback]

thanks @tswast