ENH: Effecient resample/reindex by index
Here is the idea. I have predefined index and I'm grouping by timestamps in it, aggregating preceding values of the interval:

``` python
index = DatetimeIndex([datetime(2014, 1, 11, 11, 30), datetime(2014, 1, 12, 15), datetime(2014, 1, 15, 22)])

s = Series({
    datetime(2014, 1, 10, 0): 1,
    datetime(2014, 1, 11, 10): 2,
    datetime(2014, 1, 11, 11): 3,
    datetime(2014, 1, 12, 12): 3,
    datetime(2014, 1, 15, 16): 4,
    datetime(2014, 1, 15, 22): 5
})

print s.groupby(lambda x: index[(index >= x).argmax()]).sum()
```

---

```
2014-01-11 11:30:00    6
2014-01-12 15:00:00    3
2014-01-15 22:00:00    9
```

I don't know how effecient this code, but I think it might be a very useful feature as long as "resample" accepts only offsets or deltas. And "reindex" method doesn't have 'how' argument.
