Add an alternative back-end for Unicorn

[joeyAghion]

Based heavily on Heroku::Forward::Backends::Thin, I added a Heroku::Forward::Backends::Unicorn.

Now that there are multiple back-ends, I thought it best to not automatically require all of the implementations. Instead, the proxy code must now require both heroku-forward and the chosen implementation.

The Unicorn back-end doesn't currently support the same SSL options as Thin, and adds a :config_file option referring to an optional Unicorn config file. A complete example:

require 'heroku-forward'
require 'heroku/forward/backends/unicorn'

application = File.expand_path('../my_app.ru', __FILE__)
config_file = File.expand_path('../config/unicorn.rb', __FILE__)
backend = Heroku::Forward::Backends::Unicorn.new(application: application, env: env, config_file: config_file)
proxy = Heroku::Forward::Proxy::Server.new(backend, host: '0.0.0.0', port: port, retries: 100)
proxy.logger = Logger.new(STDOUT)
proxy.forward!

[dblock]

Ruby 1.8.7 build failed.

[joeyAghion]

The just-added commit fixes support for ruby 1.8.7 and adds a stub unicorn.rb config file.

[dblock]

Merged, thank you!

[fancyremarker]

This is really nice, guys. Adds enormous value and robustness to the Heroku platform. I think you need to write it up!