ENH: timedelta should write correctly to csv
Though read_csv can't read these in yet....
see this: https://github.com/playpauseandstop/kikola/blob/master/kikola/utils/timedelta.py

```
In [58]: data
Out[58]: '  completed             deadline\n15-07-2013 23:10    15-07-2013 23:15\n16-07-2013 00:20    16-07-2013 00:15\n16-07-2013 00:20    16-07-2013 00:15\n16-07-2013 21:04    16-07-2013 21:30\n16-07-2013 21:58    16-07-2013 22:00\n16-07-2013 23:21    16-07-2013 23:15\n16-07-2013 23:21    16-07-2013 23:15\n17-07-2013 00:19    17-07-2013 00:15\n17-07-2013 00:19    17-07-2013 00:15\n17-07-2013 21:18    17-07-2013 21:30\n17-07-2013 22:07    17-07-2013 22:00'

In [59]: df = read_csv(StringIO(data),index_col=None,skiprows=1,header=None,parse_dates=[[0,1],[2,3]],sep='\s+')

In [60]: df.columns = ['completed','deadline']

In [61]: df['delta'] = df['completed']-df['deadline']

In [62]: df
Out[62]: 
             completed            deadline     delta
0  2013-07-15 23:10:00 2013-07-15 23:15:00 -00:05:00
1  2013-07-16 00:20:00 2013-07-16 00:15:00  00:05:00
2  2013-07-16 00:20:00 2013-07-16 00:15:00  00:05:00
3  2013-07-16 21:04:00 2013-07-16 21:30:00 -00:26:00
4  2013-07-16 21:58:00 2013-07-16 22:00:00 -00:02:00
5  2013-07-16 23:21:00 2013-07-16 23:15:00  00:06:00
6  2013-07-16 23:21:00 2013-07-16 23:15:00  00:06:00
7  2013-07-17 00:19:00 2013-07-17 00:15:00  00:04:00
8  2013-07-17 00:19:00 2013-07-17 00:15:00  00:04:00
9  2013-07-17 21:18:00 2013-07-17 21:30:00 -00:12:00
10 2013-07-17 22:07:00 2013-07-17 22:00:00  00:07:00

In [63]: df.dtypes
Out[63]: 
completed     datetime64[ns]
deadline      datetime64[ns]
delta        timedelta64[ns]
dtype: object

In [64]: df.to_csv('test.csv')

In [65]: !cat test.csv
,completed,deadline,delta
0,2013-07-15 23:10:00,2013-07-15 23:15:00,-300000000000
1,2013-07-16 00:20:00,2013-07-16 00:15:00,300000000000
2,2013-07-16 00:20:00,2013-07-16 00:15:00,300000000000
3,2013-07-16 21:04:00,2013-07-16 21:30:00,-1560000000000
4,2013-07-16 21:58:00,2013-07-16 22:00:00,-120000000000
5,2013-07-16 23:21:00,2013-07-16 23:15:00,360000000000
6,2013-07-16 23:21:00,2013-07-16 23:15:00,360000000000
7,2013-07-17 00:19:00,2013-07-17 00:15:00,240000000000
8,2013-07-17 00:19:00,2013-07-17 00:15:00,240000000000
9,2013-07-17 21:18:00,2013-07-17 21:30:00,-720000000000
10,2013-07-17 22:07:00,2013-07-17 22:00:00,420000000000
```

work-around (but should be handled in the datetimeblock though)

```
In [66]: df['delta'] = df['delta'].apply(pd.lib.repr_timedelta64)

In [67]: df.to_csv('test.csv')

In [68]: !cat test.csv
,completed,deadline,delta
0,2013-07-15 23:10:00,2013-07-15 23:15:00,-00:05:00
1,2013-07-16 00:20:00,2013-07-16 00:15:00,00:05:00
2,2013-07-16 00:20:00,2013-07-16 00:15:00,00:05:00
3,2013-07-16 21:04:00,2013-07-16 21:30:00,-00:26:00
4,2013-07-16 21:58:00,2013-07-16 22:00:00,-00:02:00
5,2013-07-16 23:21:00,2013-07-16 23:15:00,00:06:00
6,2013-07-16 23:21:00,2013-07-16 23:15:00,00:06:00
7,2013-07-17 00:19:00,2013-07-17 00:15:00,00:04:00
8,2013-07-17 00:19:00,2013-07-17 00:15:00,00:04:00
9,2013-07-17 21:18:00,2013-07-17 21:30:00,-00:12:00
10,2013-07-17 22:07:00,2013-07-17 22:00:00,00:07:00

```
