Resove #651: Make add_secondary_index return index

A pointer to the newly created index object is now returned.

libraries/db/include/graphene/db/index.hpp

@@ -168,6 +168,7 @@ namespace graphene { namespace db {
          void add_secondary_index()
          {
             _sindex.emplace_back( new T() );
+            return static_cast<T*>(_sindex.back().get());
          }
 
          template<typename T>