[BaseEntity] Remove offending and non-serializable SPContentTypeId member from BaseEntity.
- Any property on an Entity should be easy to serialize, that's the whole point.
- Down the road, we should implement a ContentTypeIdValue value type and its own converters so that we can expose this in an easily serializable fashion
- As a compromise, for now we keep the ContentTypeName in there
Merge branch 'develop' of https://github.com/GSoft-SharePoint/Dynamite-2010 into develop