Adds a reference to System.Json to Android and iOS projects

If the following pull request gets accepted into Xamarin.Auth, WebEx will include JSON parsing:
https://github.com/xamarin/Xamarin.Auth/pull/26

Because Xamarin.Social also includes WebEx, this commit adds a project reference.

Commit licensed under MIT/X11
Merge pull request #12 from stampsy/xamarin-pr-add-system-json-reference

Adds a reference to System.Json to Android and iOS projects