Added flow_id attribute to packets.

This allows hosts to pass along the packet to the correct flow.
Updated doctrings.

Changed indentation and formatting to mimic other files in our repo.
Fixed packet lengths and Python 2.7 compatibility

Data packets, acknowledgment packets, routing update packets, and FIN
packets have a fixed length of 1024, 1024, 64, and 64 bytes,
respectively.

PacketTypes enum was rewritten as an internal class of the general
Packet class. This removes the reliance on Python 3.4, so we can now
commit to developing on Python 2.7.
Committing pycache since I can't sync with master...
Merge branch 'master' into Packet
Merge branch 'Packet'
Merge branch 'master' of https://github.com/kexinrong/NetworkSimulator

Conflicts:
	__pycache__/Packet.cpython-34.pyc