ENH/API: add numeric_only with a default of False to numerics
for:

`min,max,sum,var,std,kurt,median,mad,prod,skew`

The big reason to do this is a performance hit when you happen to have an object column. and you don't realize it.

See: http://stackoverflow.com/questions/18701569/pandas-dataframe-mean-very-slow-how-can-i-calculate-means-of-columns-faster

`_reduce` in Frame already supports this (and Series will ignore it), so should be trvial to implement

alternatively could just pass `**kwds` thru (I think we had an issue about this earlier)
