[ISSUE-133] add go module support

[zouzias]

Fixes #133

Simply adds the go.mod / go.sum files.

After merging the PR, a contributor should push a Git tag of the form vX.Y.Z to allow versioning.

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

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What to do if you already signed the CLA

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Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
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[zouzias]

@googlebot I signed it!

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CLAs look good, thanks!

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[googlebot]

CLAs look good, thanks!

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