Added Sink for Microsoft Application Insights for Visual Studio Online

I've been using Serilog and MS's Application Insights in two different projects in the past but for a new one wanted to use both.. sooo I wrote a Sink for it. MS itself only provides NLog and log4net Appenders but both work internally the same.

I just ported the bits over to Serilog. Let me know what you think!

Cheers,
-Joerg