Tink presubmit tests and continuous integration

Tink is testing continuously with Kokoro an internal deployment of Jenkins at Google.

Continuous builds and presubmit tests are run on Ubuntu and macOS.

Presubmit tests are triggered for the pull request if one of the following conditions is met:

• The pull request is created by a Googler.
• The pull request is attached with a kokoro:run label.

Continuous builds are triggered for every new commit, and upload to Maven snapshot versions of Tink and apps.