Set up a repeatable release process.

[non]

I had to make a few tweaks to build.sbt to get things reliably building for me
in a sustainable way. Once I got that working, I set up a release.sh script
that I can successfully use to cross-build, cross-test, and cross-publish.

The following commands are useful during the release process:

    ./release.sh compile        # build along all axes
    ./release.sh test           # ensure all tests pass
    ./release.sh package        # ensure we can package everythign
    ./release.sh publishSigned  # actually publish a signed release

Note that to publish you currently have to (temporarily) modify build.sbt to
change a few variables. You also have to log into Sonatype to close/publish the
release (assuming all its checks pass).

[non]

@ashawley I tagged you on this but I plan to merge it as soon as it's green since these changes correspond to what was actually published as 1.14.1-RC1.