Server-Sent Events support

This pull request adds Server-Sent Events support.

Obviously it needs to be documented more and unit tests but I've been using in my app to send SSE messages to browsers. I left out for now JavaScriptCore bindings I'm using to emit SSE messages from JavaScript code running in my iOS app because I figured scripting support should be another layer on top of GCDWebServer.

Let me know if there are any issues and feel free to cherry-pick what you think makes sense and toss out as you see fit.