Allow different methods of correlation when using corrwith

[dsaxton]

Hi,

The corr method for DataFrames is exceedingly useful, especially since one can specify methods other than the Pearson correlation. I think it would be nice if this same argument were available when using corrwith. Perhaps there is another workaround, but right now if one wants to calculate all the Spearman (say) correlations with a specific variable, it seems necessary to calculate the full Spearman correlation matrix and subset to the column you care about.

Thanks.

[WillAyd]

Seems reasonable - PRs are always welcome

[samwaterbury]

Looks like the corr algorithms are written in Cython but corrwith uses a Python implementation of the Pearson calculation. Should be easy enough to add the other coefficient options to the corrwith method but they should probably use Cython implementations too since Spearman and Kendall are relatively complex calculations.

I can take a look at this and see if the algorithms that corr uses can also be reused by corrwith in some way, shouldn't be too tricky