Added new README, some fixes and tests
A new README was added, describing how the system should look when
complete.
Fixes:
* check the type of seq_num (needed for tests in Python 3)
* split receive and send sockets, since they need to be bound to
  different IP addresses
* fix sequence number to be unsigned 16-bit integer when generated
* explicitly return None if no packet received in DiscoverNSDP.recv()
More testing for building packet data.