Support converters across multiple Excel spreadsheets
Hi,

When using `pd.read_excel(..., sheetname=None, converters={...})` to load all data in an Excel workbook, the columns specified in `converters` gets applied to any matching column names across _all_ spreadsheets. I have a use case where I would like to use different converter functions for columns of the same name across multiple sheets.

One potential implementation could yield:

``` python
pd.read_excel("my_data.xlsx", sheetname=None, converters={
  "Sheet1": { "my_column": do_something },
  "Sheet2": { "my_column": do_something_else }})
```

Another implementation could be:

``` python
pd.read_excel("my_data.xlsx", sheetname=None, converters={
  ("Sheet1", "my_column"): do_something,
  ("Sheet2", "my_column"): do_something_else })
```

Thanks! Please let me know if I can help out with an implementation -- I'd love to contribute to Pandas!
