The backport for the .net 35

There are a few changes which allow the use the library not only on the .net40, but also on the previous version (.net35)
Also the compatibility for Source code version of the nuget package was fixed too - method 
```cs
if (string.IsNullOrWhiteSpace(param.Value))
```
was changed to the extension method
```cs
if (param.Value.IsNullOrWhiteSpace())
```