VIS: let scatter plots obey mpl color scheme (#3338)
Closes #3338.

Fixes the remaining plot functions that don't follow the matplotlib style (functions based on scatter plots).

Rationale:
- set the color keyword `c` to default one defined in matplotlibs rcParams if `c` was not specified by the user (because `c='b'` is harcoded in matplotlib `scatter` method).
- the `color` keyword is not checked because, if specified by the user, it already is given preference by matplotlib over `c` if both are given.

There is a new PR on scatter based plotting methods (#3473), but I suppose that it can be handled in that PR to ensure this new plotting method also follows this approach?

@cpcloud ?
 pulling it down now
 Still get obnoxious blue:

![funky-colors](https://f.cloud.github.com/assets/417981/1258494/61e199da-2bbf-11e3-81af-db6070f50997.png)
 @cpcloud How did you obtain this plot?

If I do (example from the docs):

``` python
pd.options.display.mpl_style = 'default'
df = pd.DataFrame(np.random.randn(1000, 4), index=pd.date_range('1/1/2000', periods=1000), columns=list('ABCD'))
df = df.cumsum()
df.plot()
```

I get the new color scheme, as you also see in the docs: http://pandas.pydata.org/pandas-docs/dev/visualization.html#basic-plotting-plot
 @cpcloud Ah, it seems like you posted the plot in this part of the docs: http://pandas.pydata.org/pandas-docs/dev/visualization.html#suppressing-tick-resolution-adjustment

Well, this one is generated by explicitly stating the colors you want (`df.A.plot(color='r')`), so it logically follows this and not the default color scheme.

Off course, we could remove this `color=..` like this:

```
with pd.plot_params.use('x_compat', True):
    df.A.plot()
    df.B.plot()
    df.C.plot()
```

to get rid of these colors in the docs, but then the purpose of the example is a little bit lost, because you could do the same with just `df[['A', 'B', 'C']].plot(x_compat=True)`
 @jorisvandenbossche ready to go?
 @cpcloud this ok?
 @jreback for me this is ready
 Yep good 2 go 
