This PR adds the ability to define variables 's' and 'z' that can be used to construct transfer functions using formulas. The matlab version of this matches MATLAB:

from control.matlab import tf
s = tf('s')
G = (s + 1) / (s**2 + 2*s + s)

The Python version uses the special variables TransferFunction.s and TransferFunction.z:

import control as ct
s = ct.TransferFunction.s
G = (s + 1) / (s**2 + 2*s + s)

The constants TransferFunction.s and TransferFunction.z are defined to be transfer functions corresponding to 's' and 'z'. The rest is taken care of by the existing operator overloading functions for transfer functions.