Support for Reading Excel Files with Hierarchical Columns Names
related #4438 #4679 

For example see the "INCOME_STATEMENT" sheet on http://shareholder.api.edgar-online.com/efx_dll/edgarpro.dll?FetchFilingXLS1?sessionid=F7D_689jk-BEFvN&ID=9277772

There are four columns in the desired DataFrame:

```
1           2               3           4
Thirteen weeks ended        Twenty-six weeks ended  
March 30,   March 24,       March 30,   March 24,
```

Currently the `ExcelFile` parser only takes an int for the header parameter, whereas the html parser takes "The row (or rows for a MultiIndex) to use to make the columns headers."
