Added libevent2-based I/O backend for NancyFx (10 times faster)

Peak performance on my hardware - 10186 req/seq
Benchmark log (JSON) - http://paste.ubuntu.com/6133922/
Host source code - https://github.com/kekekeks/Nancy.Hosting.Event.2
It's approximately 10 times faster than default Mono ASP.NET host.