[docs] upgrade UPGRADING.md
[api] export the httpProxy.Server as the main export but preserve the createServer factory
[fix] comments
Merge pull request #536 from nodejitsu/caronte-api

export the proxy itself from the main require
[fix] add `type` to before and after to grab correct `passes`, fixes #537
Merge pull request #539 from nodejitsu/fix-before-after

[fix] add `type` to before and after to grab correct `passes`, fixes #537