Moved some of the serialization classes (internal only) and added a ItemFieldConverter to convert a list of Podio.Item 's to their correct type upon de-serialization.

Fixed some regression in "breaking changes".
This version of the PCL should be backwards compatible with the exception of WebProxy

Signed-off-by: Ryan Hatfield <ryan.hatfield@onsharp.com>
Merge branch 'FourFive' of https://github.com/onsharpryan/podio-dotnet into FourFive