Introduced NullReflectionElement class.
Implemented NullReflectionElement.

This is (hopefully obviously) an implementation of the Null Object
pattern.
Documented NullReflectionElement.
Overrode Equals and GetHashCode.
Documented overriden methods.

This resolves #23 (https://github.com/ploeh/Albedo/issues/23).
Gave AssemblyElement Structural Equality.
Documented the Equals overload.
Documented overridden GetHashCode.