enh request: new functions cummax, cummin

[fabriziop]

I have found useful cummax and cummin functions in R, so I propose to add them to pandas.

At fabriziop/pandas there is a forked branch with these changes, commit 660d5ddf05c49c55fabce072a41e5d5b5096e529 .

The new functions are dumbly derived from the cumsum function.