Fix java.lang.NullPointerException when sending a packet with no payload.
Minor code cleanups to comply with the deadline.

Now the sender opens a socket, reads a file and divides it in 512B chunks,
assembles some HipsterPackets and sends them blindly over UDP.
When finished reading the file it sends an empty ETX packet to finalize the
file transfer.
Merge branch 'master' of ssh://github.com/DvdMgr/HIPSTER