Initial FAST TCP implementation

I haven't tested anything, but this commit contains the basic
implementation of FAST TCP.  I also added some TODO's to the network &
gui modules, as they will need to be modified to fully support TCP
Next expected ID's stored as min heap

When a data packet is received, HostResource used to just send back an
ACK whose next expected ID was simply the packet's ID + 1.  Now, the
expected ID's for each source flow are kept in a min heap
Removes packet generation code from Flow class

The packet generation code in Flow was meant for validation of the host
& link code (i.e., test case 0); now that we're implementing TCP, it
should be the responsibility of the TCP algorithm (class) to
generate/send packets
Adds inline comments, minor upgrades

I commented the TCP class, now that the structure is mostly final.  I
also made some minor upgrades to the TCP code & the expected ID min
heaps to handle edge cases & proper termination.  Fixed some typos &
renamed _env attributes to env (they should be publicly exposed)
Adds docstrings for FAST class
Fixes FAST runtime errors

Now that enough of FAST TCP was implemented to test, I ran some
simulations, and fixed all of the errors that I was getting.  I have yet
to investigate whether or not it behaves as expected, but more complex
test cases should be coming soon.
Adds FAST support to gui, network modules

The gui.FlowDialog now takes and correctly processes TCP parameters for
flows.  Likewise, the network module now initializes Flow's properly
again.  I also updated test.ZERO to use FAST TCP, and process.Flow's
docstring.
Merge branch 'fast_tcp' into develop