Feature request: Let reindex fill with per-column methods
Pandas.DataFrame.reindex takes the argument `method` which allows it to fill in the holes in the reindexed DataFrame. Currently it accepts one method which is used for the whole DataFrame, but sometimes you want to fill the different columns differently (e.g. rates are 0 but "state-columns" are forward filled). I propose that `method` should be expanded to take a column-name -> method dict giving the method for each column, preferably with "fill_value: Value" as one of the methods. 

Reindexing a single int Series with a fill_value ensures that the series stays int, instead of being casted to float. The new Pandas.DataFrame.reindex should have the same behaviour for columns where a method is given. 

Currently the only way I have found of doing this is to reindex each column independently and then writing them back into the reindexed DataFrame, but that is cumbersome and in-efficient. 