[BUG] to_csv() cols parameter not working properly.
<b>pandas version:</b> 0.11
python version: 2.7.4
platforms: linux & windows

When using cols parameter of to_csv() method only the order of the header is changed, while actual values are not.

``` python
import pandas as pd
from cStringIO import StringIO

data = """\
date,time,X1,X2
2013-1-1,0030,0.1,0.4
2013-1-1,0100,0.2,0.3
2013-1-1,0130,0.3,0.2
2013-1-1,0200,0.4,0.1
"""

df = pd.read_csv(StringIO(data), parse_dates=[['date', 'time']], index_col=0)
df.to_csv('data.csv', index_label='date_time', cols=['X2', 'X1'])
```

<b>Expected output:</b>
date_time,X2,X1
2013-01-01 00:30:00,0.4,0.1
2013-01-01 01:00:00,0.3,0.2
2013-01-01 01:30:00,0.2,0.3
2013-01-01 02:00:00,0.1,0.4

<b>Actual output:</b>
date_time,X2,X1
2013-01-01 00:30:00,0.1,0.4
2013-01-01 01:00:00,0.2,0.3
2013-01-01 01:30:00,0.3,0.2
2013-01-01 02:00:00,0.4,0.1

``` python
df.to_csv('data.csv', index_label='date_time', cols=['X2'])
```

date_time,X2
2013-01-01 00:30:00,0.1
2013-01-01 01:00:00,0.2
2013-01-01 01:30:00,0.3
2013-01-01 02:00:00,0.4

``` python
print df['X1']
```

date_time
2013-01-01 00:30:00    0.1
2013-01-01 01:00:00    0.2
2013-01-01 01:30:00    0.3
2013-01-01 02:00:00    0.4
Name: X1, dtype: float64

``` python
print df['X2']
```

date_time
2013-01-01 00:30:00    0.4
2013-01-01 01:00:00    0.3
2013-01-01 01:30:00    0.2
2013-01-01 02:00:00    0.1
Name: X2, dtype: float64
