Support for logging to Serilog (http://serilog.net)

Hi folks; I've been looking at using Topshelf with Serilog, a new but now fairly stable logging library that I've cooked up. Since I needed to put this support together I thought I'd post it back to you to see if you're interested in including it with Topshelf.

Since Serilog is a .NET Core library (Win8 + WinPhone8 + .NET 4.5) the Topshelf.Serilog assembly targets .NET 4.5.

I didn't look into how you're creating NuGet packages, so I haven't tested this with any kind of packaging. Thought it best to get your feedback on whether it is useful to the project before digging in much further.

I'm happy to send further pull requests to keep this up to date if needed in the future.

Anyhow, great to see Topshelf still going strong, has had a lot of use in projects I've worked on. Keep it up! -- Nick.