Update copyright records on all source files; add AUTHORS.txt and CONTRIBUTORS.txt

[mbrukman]

As we did for JanusGraph when we imported and cleaned up the Titan repo, we should make the following changes:

• [ ] "Copyright YEAR Expero Inc." should be updated to "Copyright YEAR JanusGraph Authors"
• [ ] we should create AUTHORS.txt and list the copyright holders there — starting with "Expero Inc." in this case¹
• [ ] we should create CONTRIBUTORS.txt and list the contributors (non-copyright-holders) there — starting with Ted Wilmes in this case²

For example, in case when a developer is working for a company, and the company retains copyright over their contributions to JanusGraph, the company would be listed in AUTHORS.txt while the person contributing should be in CONTRIBUTORS.txt. You can see examples of these files in the JanusGraph repo:

• https://github.com/JanusGraph/janusgraph/blob/master/AUTHORS.txt
• https://github.com/JanusGraph/janusgraph/blob/master/CONTRIBUTORS.txt

The CONTRIBUTORS.txt includes a command for how the initial version was generated, so we can start with that.

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¹ and any other historical contributors who own copyrights over their contributions, or their companies if they own those contributions ² and any other historical contributors who don't own copyright over their contributions — if they do, they should be listed in AUTHORS.txt instead