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.
What does this test?

Really, this should inspect the resulting Line2D and assert that its colour is that defined in rcParams (maybe change the rcParams before to make it distinct).