Add list of authors and contributors

[mbrukman]

We need to keep and maintain a list of all authors (copyright holders, which may be individuals or corporations) and contributors (individuals who actually did the work).

For an example, see Go authors and contributors. Some projects conflate the two categories, which makes it unclear who owns the copyright vs. who contributed effort to the project.

We need to be precise in this regard, because our copyright statement will be something akin to:

Copyright 2017 JanusGraph Authors

so we need to list the authors and not just contributors, who may have signed away their copyright to someone else or an organization.

Note, however, that the Go project re-uses the list of contributors also as a list of folks who are eligible to contribute due to having signed the CLA, but not who specifically contributed. We will not be making the list of CLA signatories public, but only the list of folks who actually contributed to the project.

[mbrukman]

Fixed in PR #25.