DataFrame.plot error when both 'color' and 'style' arguments are passed (GH9671)
closes #9671

`self.style` can be a list of strings or a string. If it is a string, we make a list containing that string, so that we only have to deal with a list of strings. We check every element in that list and raise an error if it has a color symbol.
The test makes sure that calling `plot()` and passing both `color` and `style` arguments is allowed if there is no color symbol in the style string(s), and that passing both `color` and `style` arguments is not allowed if there is a color symbol in the style string(s). This test is done for a dataframe and for a series.
