BLACK-142: Stunnel Heroku buildpack

I'm opening this here because that's where I've been developing it. Once this is merged, I will squash it back to an initial commit and a new repo in the Yesware org (FYI, this needs to be public for it to work).

This is the result of:
* Cloning the PgBouncer Buildpack
* Deleting all git history
* Opening this PR with all the changes I did from one to the other

I did that because I figured it would make some of the "Why did you do it this way?" questions easier, since the answer will most often be "Because that's how it was in the PgBouncer build pack"

Simply put, this build pack installs stunnel into the dyne and configures it at runtime. It pulls the certificate from an ENV var and drops it on disk. It also iterates over listed ENV vars holding Redis URLs and maps them to localhost and custom ports. See the README for an example.