Fixing compiler warnings, mostly related to unused parameters.

Thanks to David Hotham for the patch. Fixes issue 8.

pb.h

@@ -17,6 +17,11 @@
 #define pb_packed
 #endif
 
+/* Handly macro for suppressing unreferenced-parameter compiler warnings.    */
+#ifndef UNUSED
+#define UNUSED(x) (void)(x)
+#endif
+
 /* List of possible field types. These are used in the autogenerated code.
  * Least-significant 4 bits tell the scalar type
  * Most-significant 4 bits specify repeated/required/packed etc.