Update the rest of the extensions to support .netstandard

• NInject
• SimpleInjector
• StructureMap

Thanks for the work on the great library. I am having trouble when removing objects from the DbContextMock when they are related to one another. For example, I have two classes, one being a parent to the other. I create the mock sets with multiple children related to a single parent, like so:

var children = new Children[] { new Child { Id = 1, ParentId = 1, ... }, new Child { Id = 2