Running examples from fresh checkout fails

Icarus doesn't create the `vcd` directory, so running any of the examples from a fresh checkout fails with the following error:

```
compiling ...
cosimulation setup ...
VCD Error: tb_vgasys.v:17: Unable to open vcd/_tb_vgasys.vcd for output.
Traceback (most recent call last):
  File "test_vgasys.py", line 129, in <module>
    test_vgasys(Namespace())
  File "test_vgasys.py", line 73, in test_vgasys
    bvga, cvga, mvga)
  File "test_vgasys.py", line 49, in _prep_cosim
    mm_pxlen=mvga.pxlen, mm_active=mvga.active
  File "/usr/lib/python2.7/site-packages/myhdl/_Cosimulation.py", line 88, in __init__
    raise CosimulationError(_error.SimulationEnd)
myhdl.CosimulationError: Premature simulation end
```

These changes create the `vcd` directory during `_prep_cosim` and also create the `plots` directory for `ex4_mathsop`.  Note I haven't updated `ex5_medfilt` since this fails for another reason and therefore I can't test the change.