BUG: Corrupted excel output when zero in top left of DataFrame
When DataFrame().values[0,0] is a 0 integer, the excel output is corrupted.
When opening the excel file in Microsoft Excel 2010 I get:

```
Excel found unreadable content in 'a.xlsx'.
Do you want to recover the contents of this workbook?
If you trust the source of this workbook, click Yes.
```

When I click Yes, I get:

```
Excel was able to open the file by repairing or removing the unreadable content.
Removed Part: /xl/sharedStrings.xml part with XML error. 
(Strings) Illegal xml character. Line 1, column 95.
Removed Records: Cell information from /xl/worksheets/sheet1.xml part
```

The cell that should have been 0 is now empty.

Here is an example:

``` python
import pandas
import numpy as np
a = np.arange(25).reshape(5,5)
pandas.DataFrame(a).to_excel('./a.xlsx') # this is 'unreadable'
pandas.DataFrame(a.astype(float)).to_excel('./afloat.xlsx') # this is OK
a[0,0] = 1
pandas.DataFrame(a).to_excel('./a1.xlsx') # this is OK
```

Packages: (from 'pip list' as I couldn't find [ci/print_versions.py](https://github.com/pydata/pandas/blob/master/ci/print_versions.py) mentioned in [CONTRIBUTING](https://github.com/pydata/pandas/blob/master/CONTRIBUTING.md))
pandas (0.12.0)
xlrd (0.9.2)
openpyxl (1.6.2)
