http_app: limit export from syntax module

Limit the names the netkat.syntax module exports when used in the
following way:

   from netkat.syntax import *

This will hide class definitions so that class instances can only be
constructed and manipulated through the exported functions and
overloaded builtin operators.
http_app: add lang/py/examples directory, null example

The examples directory doubles as a python module so that developers can
run examples in the following way:

  $ python -m examples.null

In order for this to work, the example applications should have a block
of code below any definitions that looks like this:

  if __name__ == '__main__':
      netkat.flaskapp.create(state, handler).run()
http_app: python project files and README