ENH: Better handling of MultiIndex with Excel
I'm going to put something together this weekend, even if I can only make it work with one engine. The current MI output for columns is too hard to use. Would be really nice to round-trip it, but I'm less concerned about that.

@jmcnamara if you have any tips they'd certainly be appreciated - I may only do it for Xlsxwriter this weekend. (existing  MI summary form can judge where to put whitespace to show that one index covers multiple values. Just need to do the same thing but rather than blank cells, should be a merged cell).

@cancan101 were you interested in this too?
