Added Doxygen Modules describing the Object Helper Functions (and how they're used) and the Handler-to-Objects interface functions of the Device Object.

Two small code corrections in device.c:
a) Fixed the return value of Device_Write_Property() so that a -1 value would also return False.
b) Fixed the string length checked when writing PROP_OBJECT_NAME for the Device object.

bacnet-stack/include/wp.h

@@ -58,6 +58,7 @@ typedef struct BACnet_Write_Property_Data {
 /** Attempts to write a new value to one property for this object type
  *  of a given instance.
  * A function template; @see device.c for assignment to object types.
+ * @ingroup ObjHelpers 
  *
  * @param wp_data [in] Pointer to the BACnet_Write_Property_Data structure,
  *                     which is packed with the information from the WP request.