Update nrf24.py

Fixed problem with write_payload   
 - buf extended instead of txbuffer 
Added buf_len parameter to read_payload  
 - If set as -1 the complete payload will be put into buf read_payload will now return read bytes count 
Fixed problem in write  
 - status is now filled with actual status and not OBSERVE_TX
Removed \r\n in print_status
Merge pull request #5 from Angus71/master

Improve/fix tx and rx processes