BUG: Excel writer doesn't handle "cols" option correctly
This is an issue that I introduced when fixing #5235.

The `cols` option in `to_excel` no longer works correctly after the fix for the above issue.  For example:

``` python
import pandas as pd

df = pd.DataFrame({'A': ['a', 'a', 'a'],
                   'B': ['b', 'b', 'b']})

df.to_excel('frame.xlsx', sheet_name='Sheet1', cols=['B', 'A'])

```

Gives:

![screenshot](https://f.cloud.github.com/assets/94267/1462277/a4f1cd20-44ed-11e3-96be-993c49f342a6.png)

Note, the headers are changed but not the column data.

I have a proposed fix and test for this. Should I create a new branch/PR or merge it into the Excel MultiIndex PR: #5423
