Added failing test:
Apply_Changes_Should_Mark_Unchanged_Order_Modified_Customer_With_Addresses_Multiple_Added

Throws InvalidOperationException:
Attaching an entity of type 'TrackableEntities.EF.Tests.NorthwindModels.CustomerAddress' failed because another entity of the same type already has the same primary key value.
Fixed DbContextExtensions.ApplyChanges so that test now passes:
Apply_Changes_Should_Mark_Unchanged_Order_Modified_Customer_With_Addresses_Multiple_Added
Added failing test:
Apply_Changes_Should_Mark_Unchanged_Order_Deleted_Customer_With_Addresses_Multiple_Added

Throws InvalidOperationException:
Attaching an entity of type 'TrackableEntities.EF.Tests.NorthwindModels.CustomerAddress' failed because another entity of the same type already has the same primary key value.
Fixed failing test:
Apply_Changes_Should_Mark_Unchanged_Order_Deleted_Customer_With_Addresses_Multiple_Added

Added an exception handler to DbContextExtensions.ApplyChanges for when a Deleted parent has multuple added children, which throws an InvalidOperationException stating this condition is not allowed.
Added test:
Apply_Changes_Should_Mark_Modified_Order_Unchanged_Customer_With_Addresses_Multiple_Added, plus additional related tests.
Corrected failing test for EF5:
Apply_Changes_Should_Mark_Unchanged_Order_Deleted_Customer_With_Addresses_Multiple_Added.

Removed condition in catch for specific error message, because it would not work for localized messages.