Support optional Apex Bindings

[afawcett]

Following my last blog and the code samples in its summary.... I am wondering if an option for the Apex Injector to optionally require a binding if the binding reference (the class) is in-fact also the implementation. This would support simple cases where the developer just wants to use the Injector to add injection for a one off type thats not subclassed., but is a dependency they want to mock.