[Twisted-Python] flow is now in twisted

[Clark C. Evans]

As of this early morning, the flow module has moved from the
sandbox into twisted.flow; for those using the sandbox version,
simply change:

   import flow

to

   from twisted.flow import flow


The only change was to break flow.py into several chunks

   flow/base          exceptions and abstract base classes

   flow/stage         public stages such as Zip, Merge, Callback, etc.

   flow/controller    controllers, such as Block and Deferred

   flow/threads       threading support and QueryIterator

   flow/protocol      makeProtocol and Protocol

However, to make it easy, all of the public classes in
the above files have been imported into a 'mega' module:
 
   flow/flow.py          This is just a file which imports
                         public objects from the other files.


On the TODO list includes:

   1.  Link flow to what ever the next generation Consumer/Producer
       interfaces are so that flow just slips in.

   2.  Add more to flow/protocol to support various sorts of
       filters on protocols (such as an equivalent to LineReader)

   3.  More documentation (can there ever be enough?)

   4.  A "C" version, when the time is ripe; this will mostly
       focus on two functions: Stage.next() and _Iterator._yield()

In the next 3 months I won't have much time to throw at this, but
I am always open to suggestions and improvements.

Extra thank-yous to Etrepum, Itamar, Exarkun, Radix, Glyph, and Pahan.

Share and Enjoy!

Clark