Allow `color=None` to be passed to plotting functions.

I came across the situation where I wanted to possibly pass the color to `axes.plot`, but in the cases where I didn't want to pass the color, I wanted to pass `color=None`.  Issuing the command
```python
plot(linspace(0,1,100),color=None)
```
leads to 
```python
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/figure.py", line 1034, in draw
    func(*args)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/axes.py", line 2086, in draw
    a.draw(renderer)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/lines.py", line 529, in draw
    ln_color_rgba = self._get_rgba_ln_color()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/lines.py", line 1038, in _get_rgba_ln_color
    rgba_ln = colorConverter.to_rgba(ln_color, self._alpha)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/colors.py", line 365, in to_rgba
    'to_rgba: Invalid rgba arg "%s"\n%s' % (str(arg), exc))
ValueError: to_rgba: Invalid rgba arg "None"
to_rgb: Invalid rgb arg "None"
cannot convert argument to rgb sequence
```

Because this is such a small change, and I have not built matplotlib from scratch before, I have not verified that the coding introduced by this pull request will actually do what I want.  If the change is not benign enough and straightforward enough, then a developer can let me know that I need to check things out.