completed ARP_Beispiel.groovy and added threading to ARP.groovy

ARP_Beispiel.groovy now instantiantes an ARP object, waits three seconds,
and then uses it to send an ARP request to the IP specified with the -t
command line parameter. Example usage:

  ARP_Beispiel.groovy -t 192.168.1.1

The instantiated ARP object will print relevant information to STDOUT
(ARP requests and replies received) and will answer ARP requests sent
for our IP automatically.

If ARP_Beispiel.groovy is invoked with the command line parameter -d
it will print (a lot of) debugging information concerning the packets
received from JPCAP.

NOTE: I did not test the static methods of the ARP class extensively.
      My ARP request received a correct reply and ARP requests to the
      computer where the program was running were also correctly
      answered (twice, once by the OS and once by the program), so
      I assume they are working.