multiindex column in to_excel
link to #1651

Saving a multiindex column to_excel saves a sparse index.

Migrated from this [StackOverflow question](http://stackoverflow.com/questions/14341584/exporting-pandas-dataframe-with-multiindex), with a smaller DataFrame.

```
In [1]: m = MultiIndex.from_tuples([(1,1),(1,2)], names=['a','b'])

In [2]: df = DataFrame([[1,2],[3,4]], columns=m)

In [3]: df
Out[3]: 
a  1   
b  1  2
0  1  2
1  3  4

In [4]: df.to_excel('test.xls')
```

Saves the xls:

```
    1.1 .2
0   1   2
1   3   4
```

This differs from how `to_csv` (which is not sparse):

```
,"(1L, 1L)","(1L, 2L)"
0,1,2
1,3,4
```
