Added -P port, so that we can test on different ports

We may want to do DHCP testing on a "live" network, and not
break existing DHCP clients and servers.
Added giaddr option
Added -S for server address

packets are still sent from 0.0.0.0 for now
Merge pull request #2 from alandekok/master

Add options for destination port, giaddr, and dhcp server address