Move the CSF.Reflection.EnumExtensions.IsDefinedValue to CSF namespace.

This method did not really belong in the 'Reflection' namespace and new
functionality is about to be added to an EnumExtensions type in the base
namespace - so this seems like a sane thing to do.
Resolve #16 - Add 'HasFlag' enumeration extension method.

This analyses an enumeration value and determines whether or not it contains
a given flag value.

* Tests added

* Two sister methods of AddFlag and RemoveFlag were dropped from this issue, as
  they are too complex to implement properly.  If they are desired then they can
  be added into a new issue of their own.