Initial check in of the Split "Core" PCL solution for the Windows Project

Created new MonoGame.Framework PCL library - assembly name MonoGame.Framework
Updated Windows solution to reference new "Core" project and excluded all types managed by "Core" project
Updated Windows Project assembly name to MonoGame.Framework.Platform
Removed unnessasary usings from Shader/ShaderStage.cs (only change)
Updated Content/ContentTypeReaderManager.cs to allow resolution of types from the "Core" Project

No other files have been changed or updated as part of this initial PR
Fully tested against a windows project