closed='both' option for resample
When I call resample to downsample a certain time series and aggregate the in-between values, sometimes I want both borders included in the aggregation bin (repeating the borders in two neighbouring bins).

This might be offered as a `closed='both'` option for `resample`.

An example when this might be necessary is e.g. numerical integration

```
from scipy import integrate
def ts_integrate(ts):
    print ts
    return integrate.simps(ts, ts.index.astype(np.int64) / 10**9)

ts2 = pd.Series(1, pd.date_range(start='2012-01-23', periods=12, freq='1s'))
ts3 = ts2.resample('3s', how = ts_integrate, closed='left')
```

Elements that enter the 1st bin are

```
2012-01-23 00:00:00    1
2012-01-23 00:00:01    1
2012-01-23 00:00:02    1
```

and `2012-01-23 00:00:03` ends up **only** in the 2nd bin. This results in the integral for the first bin equaling 2, whereas it should be 3 (the `00:00:02`-`00:00:03` interval isn't processed by anybody).

With the `closed='both'` option, bins would be

```
2012-01-23 00:00:00    1
2012-01-23 00:00:01    1
2012-01-23 00:00:02    1
2012-01-23 00:00:03    1

2012-01-23 00:00:03    1
2012-01-23 00:00:04    1
2012-01-23 00:00:05    1
2012-01-23 00:00:06    1
```

resulting in the wanted `[3, 3, ...]` integral values. 

If the patch is not too difficult and someone points me in the right direction, I might try to do it myself.
