migrating back to .net 4 / vs 2010

Altered csproj files and nuget references to go back to vs 2010 / .net
4.0
Applied the changes in
https://github.com/etishor/Metrics.NET/commit/9a64bad04b861dc6c91087511df87f091121784c
(see https://github.com/etishor/Metrics.NET/issues/3) where necessary.
The OWIN code is not ported back as it uses async / await.