ExcelFile parse of empty sheet fails with "IndexError: list index out of range"
Using pandas 0.13.1 on OS X Mavericks to parse a blank Excel spreadsheet causes "IndexError: list index out of range". Apparently the default header=0 in `_parse_excel` causes the execution of `_trim_excel_header(data[header])`. Perhaps when nrows==0 this should not be executed.

``` python
import pandas as pd
xl_file = pd.ExcelFile('blank.xlsx')
xl_file.parse('Sheet1') #Sheet1 has no data
```

STDERR:

```
Traceback (most recent call last):
  File "/Users/myourshaw/lab/pypeline/python2/excel_example.py", line 10, in <module>
    xl_file.parse('Sheet1')
  File "/usr/local/Cellar/python/2.7.6/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/io/excel.py", line 208, in parse
    **kwds)
  File "/usr/local/Cellar/python/2.7.6/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/io/excel.py", line 291, in _parse_excel
    data[header] = _trim_excel_header(data[header])
IndexError: list index out of range
```
