Added EventLogSink and added lines to Demo.Program to show how to use it...

As per issue: https://github.com/serilog/serilog/issues/83 I have created a new project for writing to the Windows Event Log.  It's pretty simple and I've used the EventLog class, rather than any of the fancy ETW stuff (http://blogs.msdn.com/b/vancem/archive/2012/08/13/windows-high-speed-logging-etw-in-c-net-using-system-diagnostics-tracing-eventsource.aspx).
It seems to do the job, though I've had issues with custom log names (see the remarks on the EventLog.cs class).
I've done my best to follow the existing design, but I'm guessing how a new sink should be developed, feel free to explain where I've gone wrong/ could do things better.