add IsSingleton to OT's application API (#398)

include/openthread.h

@@ -226,6 +226,15 @@ ThreadError otThreadStart(void);
  */
 ThreadError otThreadStop(void);
 
+/**
+ * This function indicates whether a node is the only router on the network.
+ *
+ * @retval TRUE   It is the only router in the network.
+ * @retval FALSE  It is a child or is not a single router in the network.
+ *
+ */
+bool otIsSingleton(void);
+
 /**
  * This function pointer is called during an IEEE 802.15.4 Active Scan when an IEEE 802.15.4 Beacon is received or
  * the scan completes.