Add a voltage controlled switch

The switch can be used with the `VON=XXX VOFF=XXX` or `VT=XXX VH=XXX` syntax.

The switching is modeled with a tanh(x) function.

The complementary current-controlled switch is missing, but it could be implemented with a voltage-controlled switch having a resistor of opportunely selected value connected across the source port.

The limited testing I did gave promising results.

Eg. 
```
BUCK CONVERTER 
* Borrowed from from http://www.ecircuitcenter.com/Circuits/smps_buck/smps_buck.htm
* Circuit drawing with nodes: http://www.ecircuitcenter.com/Circuits/smps_buck/image002.gif
* With added ESR to C1 (node 4)
*
* Simulation command with options:
*./ahkab.py cir_switch.ckt -v 6 -o buck.data --t-max-nr 1000 --t-fixed-step -t implicit_euler
*
* SWITCH DRIVER 
VCTRL	10	0	TYPE=VDC VDC=0 TYPE=PULSE V1=0 V2=5 TD=0 TR=0.1U TF=0.1U PW=5U PER=20U
*R10	10	0	1MEG
*
* INPUT VOLTAGE
VIN	1	0	TYPE=VDC	VDC=5
*
* CONVERTER
SW1	1 2	10 0 	SWM
D1	0	2	DSCH
L1	2	3	50U
C1	3	4	25U 	IC=0
RESR	4	0	.1
*
* LOAD
RL	3	0	5
*
*
.MODEL	SW SWM VON=5 VOFF=0 RON=0.01 ROFF=10MEG
.MODEL D DSCH IS=0.0002 RS=0.05 CJ0=5e-10
*
* ANALYSIS
.OP
* RUN A TRAN, USING OP AND THE IC FROM C1 AS STARTING POINT
.TRAN TSTEP=.1U TSTOP=400U UIC=2
*
* PLOT THE RESULTS
.PLOT TRAN V(2) V(3) I(L1)
.END
```

Gives as output:

![buck](https://f.cloud.github.com/assets/5038142/1067459/57f144e0-13d0-11e3-9a3d-899aa4d0f130.png)