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.cpp

@@ -51,12 +51,6 @@ Duration::Duration (const std::string& input)
   parse (input);
 }
 
-////////////////////////////////////////////////////////////////////////////////
-Duration::operator int ()
-{
-  return (int) mDays;
-}
-
 ////////////////////////////////////////////////////////////////////////////////
 Duration::operator time_t ()
 {