Add ReSharper external annotations to log methods in the Akka assembly

This makes ReSharper understand that for example log.Error's first parameter is a format string and that the method behaves just like `string.Format()`

There are two ways to annotate code so ReSharper understands it. One is to include the Resharper Annotations nuget package (which adds a c# source file to the project), but there seems to be [a lot of problems](http://stackoverflow.com/questions/24805040/how-to-ignore-reference-to-jetbrains-annotations-dll-in-nodatime-nuget-package-x) with that approach, so I decided to use the second approach: an external xml-file. It will end up in the nuget package next to the dll, and Resharper will pick it up automatically.

For more info, see
https://www.jetbrains.com/resharper/webhelp/Code_Analysis__External_Annotations.html
https://www.jetbrains.com/resharper/webhelp/Code_Analysis__String_Formatting_Methods.html