license: Switching license to GPL 3.0

Change-Id: I6b1ceb6fb6498503c4d0ddcab2ea77ffe6ad441b
Refs: #1540
diff --git a/AUTHORS.md b/AUTHORS.md
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index 0000000..4b9e531
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+Shuo Chen             <chenatu2006@gmail.com>
+Alexander Afanasyev   <http://lasr.cs.ucla.edu/afanasyev/index.html>
+Junxiao Shi           <http://www.cs.arizona.edu/people/shijunxiao/>