Allow dropping (without losing) and adding levels to MultiIndex

[gerigk]

I asked for a way to do it on http://stackoverflow.com/questions/11264307/adding-levels-to-multiindex-removing-without-losing

similar to reset_index I want to be able to drop a level from a MultiIndex that then enters the columns again. similarly I want to be able to move a column to the MultiIndex.

[wesm]

Pushed via #1577

[gerigk]

great work!

[changhiskhan]

Thanks!