scatter_matrix bug
Currently scatter_matrix passes all unmatched keyword arguments to both scatter and line subplots (off-diagonal plots are scatter, diagonal plots are discrete or continuous histograms). I needed to color the points on the scatter plots but could not pass a "c" argument or pandas would try to pass the "c" argument to the line plots and barf. This change scratches my itch and doesn't break backward compatibility.

Great. This will close #3267.
 Please add a note to RELEASE.rst, will merge for 0.11.1
