Got ack payloads working. Renamed from 'ackpacket'

RF24.h

@@ -21,7 +21,8 @@ private:
   uint8_t ce_pin; /**< "Chip Enable" pin, activates the RX or TX role */
   uint8_t csn_pin; /**< SPI Chip select */
   uint8_t payload_size; /**< Fixed size of payloads */
-  boolean ack_packet_available; /**< Whether there is an ack packet waiting */
+  boolean ack_payload_available; /**< Whether there is an ack payload waiting */
+  uint8_t ack_payload_length; /**< Dynamic size of pending ack payload. Note: not used. */
 
 protected:  
   /**
@@ -98,7 +99,17 @@ protected:
    * @return Current value of status register
    */
   uint8_t read_payload(void* buf, uint8_t len) ;
-public:
+
+  /**
+   * Read the payload length
+   *
+   * For dynamic payloads, this pulls the size of the payload off
+   * the chip
+   *
+   * @return Payload length of last-received dynamic payload
+   */
+  uint8_t read_payload_length(void);
+
   /**
    * Empty the receive buffer
    *
@@ -112,7 +123,7 @@ public:
    * @return Current value of status register
    */
   uint8_t flush_tx(void);
-protected:
+
   /**
    * Retrieve the current status of the chip
    *