ENH/BUG: allow timedelta resamples
- [x] docs

```
In [4]: pd.set_option('max_rows',10)

In [5]: df = DataFrame({'A' : np.arange(1480)},index=pd.to_timedelta(np.arange(1480),unit='T'))

In [6]: df
Out[6]: 
                    A
0 days 00:00:00     0
0 days 00:01:00     1
0 days 00:02:00     2
0 days 00:03:00     3
0 days 00:04:00     4
...               ...
1 days 00:35:00  1475
1 days 00:36:00  1476
1 days 00:37:00  1477
1 days 00:38:00  1478
1 days 00:39:00  1479

[1480 rows x 1 columns]

In [7]: df.resample('30T',how='sum')
Out[7]: 
                     A
0 days 00:00:00    435
0 days 00:30:00   1335
0 days 01:00:00   2235
0 days 01:30:00   3135
0 days 02:00:00   4035
...                ...
0 days 22:30:00  40935
0 days 23:00:00  41835
0 days 23:30:00  42735
1 days 00:00:00  43635
1 days 00:30:00  14745

[50 rows x 1 columns]
```
