DOC: groupby by indexer to 'resample' data
Related to [this question](http://stackoverflow.com/questions/37396264/pandas-equivalent-of-resample-for-integer-index).

I would like to propose either to expand `.resample` to work on indexes that are integers or to create another method that does the same thing (maybe more general `.upsample` and `.downsample` methods).

If it was implemented as an expansion of resample I imagine something like:

```
df = pd.DataFrame(np.random.randn(10,2))
df.resample(5, np.std)
```

Which would produce:

``` python
          0         1
0  1.184582  0.492113
5  0.533134  0.982562
```

which is a pretty forward downsample case. For upsampling maybe some discussion is needed on how to return the indexes.

This should be simple to program and I think it would be a very useful shortcut.
