A sparrow's nightmare: nest foreclosure... (Replace Properties nested closure with Entity nested closure)

As per design meeting notes (https://github.com/aspnet/EntityFramework/wiki/Entity-Framework-Design-Meeting-Notes-June-5,-2014#fluent-api) this change removes the ModelBuilder.Properties methods and replaces them with more general-purpose overloads of the Entity methods. This allows all the configuration for a given entity type to be placed in a nested closure.