Bug: no ecoding setting for xls.parse
Hello,
my excel file contains units in a row:

"m²", "°", etc.

It appears that this generates unicode errors:

```
UnicodeDecodeError: 'utf8' codec can't decode byte 0xb0 in position 339: invalid start byte
```

Please see the discussion / documentation here:
- https://groups.google.com/forum/#!searchin/openpyxl-users/UnicodeDecodeError/openpyxl-users/dH0Vbkwuo_w/-fbN1nHifK0J
- http://openpyxl.readthedocs.org/en/latest/api.html?highlight=encode

So my suggestion would be to incorporate an encoding option in the openpyxl reader.
But I cannot see that it's used in:
https://github.com/pydata/pandas/blob/master/pandas/io/excel.py
