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.

Thanks. In the future, you don't need to open up a new PR. You can push changes to your original pull request and github will pick them up. I made a couple comments inline. Please let me know if you've got any questions or if anything I wrote doesn't make sense.

The tests look great! I'm glad you included one that _should_ raise an exception (I always forget these). I just had a few things to make them fit into our test framework.

Once you make those changes you can test these specific ones with `nosetests test_graphics:TestDataFramePlots.<test_name>`. You also might want to make new tests instead of attaching them to `test_plot`. That way if they fail in the future we can easily see which test failed.
 Thanks for your comments Tom. I made the changes you suggested.
 @davidbrochart could you rebase on master and add a release note in `doc/source/whatsnew/v0.16.1.txt`? Thanks.

You can also [squash](https://github.com/pydata/pandas/wiki/Using-Git#fetch-and-then-rebase-interactively-to-squash-reword-fix-otherwise-change-some-commits) your commits down to one. Let me know if you have any troubles.
 @TomAugspurger I rebased on master and added a release note. Hope this is fine.
 Merged via https://github.com/pydata/pandas/commit/f00d6bba5a7ee42569b9d6f7848962fd04a3360d

Thanks @davidbrochart 
