ENH: Pass **kwargs through to matplotlib .scatter()

Allow for more configurability to making scatter plots.  Most of the other plotting functions already do this, just giving .scatter_plot() the same power.  Works for both single scatters and grouped scatters.

I didn't make a test because anything that would be tested would be matplotlib specific, but I'd be happy to do so if you'd like (test_graphics.py: TestDataFramePlots.test_scatter is a good spot).