Scatter matrix labels fix
fix scatter matrix axis labels for odd number of columns

https://github.com/pydata/pandas/issues/2756

oops - included my last fix in here too. I can fix that if it is a problem - just let me know.
 no need, I'll do it. Thanks for pitching in!
 A I missing something? the example in #2756 looks unchanged.
 I only added a C label the first try. Just took another look - the axis labels should be complete now. Here's an example: http://nbviewer.ipython.org/5189208
 Thanks, I rebased and merged. Opted for the traditional tick arrangement.

We could use more help sorting out the plotting part of pandas, there
are a bunch of open issues if you feel like tackling some more stuff.

I opened #3075 yesterday, just need to figure out a way to bake in a color scheme
without messing people's own customizations. The current scheme is inspired by 
1992 CGA screens, we could do better...
