.net35 backport

I successfully moved main package and several
* Serilog.Sinks.EventLog
* Serilog.Sinks.Log4Net
* Serilog.Sinks.MSSqlServer

package to the .net35. I know that this is really old framework, but sometimes people should use it :-1: 

The main difference - there is no support for Rx Extensions for .net35. Right now I don't how to save this functionality in the .net35.