Added a `DataTypeGuid` property to the `Property` object.

This is required for Courier's DataResolver to "hot-swap" the DataTypeId (int) with it's unique Id (Guid).
Merge branch 'master' into feature-patch-courier-dataresolver
Changed the `Property` properties to `internal` setters.

Made the `DataTypeGuid` property completely `internal`.
Exposed the `internal` members to the "Archetype.Courier" (and Test) assemblies.
Merge pull request #58 from leekelleher/feature-patch-courier-dataresolver

Added a `DataTypeGuid` property to the `Property` object.