System modules when using Forms and a PCL project

Hi,

I am developing an app for Android and iOS which uses oAuth and would like to do this as a PCL project and Xamarin.Forms.
What i cannot seem to figure out how to use System.Json and System.Security.Cryptography in the PCL project of the solution.

When I include the files in the .iOS or .Android projects they build fine.
When I include those same files in the PCL I am getting errors like:
Error CS0246: The type or namespace name

HttpClientHandler' could not be found. Are you missing an assembly reference?
or 
Error CS0234: The type or namespace name

I do include the 'using System.Json' and 'using System.Security.Cryptography' so i do not understand the errors I am getting.
Can anyone help me with this?