Create visitor API for AssemblyElement
Use Data Flow Verification,

as described in
http://blog.ploeh.dk/2013/10/23/mocks-for-commands-stubs-for-queries/
Removed ReflectionVisitor,

because there are no tests, and it complicates the PR.
Relaxed restriction on visitor element reference,

and removed the visitor parameter from DelegatingReflectionVisitor<T>