Refactor to extract functionality from Sweetness

* Use ERB for server.conf
* Process list of UPSTREAM_SERVERS passed via ENV
* Ensure new SSL certificates each run

Fixes #11
Correctly remove temporary test key/certificate
Add FORCE_SSL option to automatically redirect all requests to HTTPS
Merge pull request #20 from fancyremarker/env-upstreams

Refactor as fully-functional reverse proxy