* test that hiding secret with expired key fails
* add test of user key without username.
* revert to performing most tests with 'user1'.
* move user4 private/public key fixtures
* factor code fetching emails from keyrings, add comments.
* use factored _warn_or_abort()
* add to, clean up, and clarify comments.
Changes:
1. Now everything is tested inside the `docker`-containers and `OSX` images on `travis`.
2. We now have `CONTRIBUTING.md` and `LICENSE.md`. `README.md` is also changed.
3. We have a brand logo.
4. We have autodeploy to `bintray`.
5. Everything is `shellcheck`ed (except `tests/`).
Closes#32#33#34#35#39