PERF: write basic datetimes faster
with PR

```
In [2]: df = DataFrame({'A' : pd.date_range('20130101',periods=100000,freq='s')})

In [3]: %timeit df.to_csv('test.csv',mode='w')
1 loops, best of 3: 282 ms per loop
```

0.16.1

```
In [2]: %timeit df.to_csv('test.csv',mode='w')
1 loops, best of 3: 352 ms per loop
```
