Bug Fix - #231 time_t/int collision on 32-bit systems

- Fixed bug #231 by removing the operator int () cast from the Duration
  class.  On a 32-bit system, "time_t" may be defined as "int", in
  which case the int cast and time_t cast are essentially duplicates.
  Thanks to Pietro Cerutti, who also provided a patch.

src/Duration.h

@@ -40,7 +40,6 @@ public:
   bool operator> (const Duration&);
   ~Duration ();                          // Destructor
 
-  operator int ();
   operator time_t ();
   operator std::string ();