Installing lxml causes xlsx exports to be ill-formed
I noticed xlsx files I was exporting from Pandas with to_excel weren't opening in Numbers and Preview on my Mac. They did, however, open in Excel and Google Drive. 

I created a new virtualenv and installed Pandas and openpyxl with pip:

```
numpy==1.8.1
openpyxl==1.8.6
pandas==0.13.1
python-dateutil==2.2
pytz==2014.2
six==1.6.1
wsgiref==0.1.2
```

Running this code creates a xlsx file that seems to be ok (opens in Numbers anyway):

```
>>> import pandas as pd
>>> df = pd.DataFrame.from_records([[1,2],[3,4]], columns=["a","b"])
>>> df.to_excel("without_lxml.xlsx")
```

However, this same code was creating ill-formed xlsx files in my project. I narrowed the culprit down to lxml. I installed lxml in this fresh venv so `pip freeze` gives:

```
lxml==3.3.5
numpy==1.8.1
openpyxl==1.8.6
pandas==0.13.1
python-dateutil==2.2
pytz==2014.2
six==1.6.1
wsgiref==0.1.2
```

At that point, I ran the same code and the new file (with_lxml.xlsx) doesn't open in Numbers or Preview.

```
>>> import pandas as pd
>>> df = pd.DataFrame.from_records([[1,2],[3,4]], columns=["a","b"])
>>> df.to_excel("with_lxml.xlsx")
```

You can see the Excel files created:
- http://stiglerimages.s3.amazonaws.com/dropzone/without_lxml.xlsx
- http://stiglerimages.s3.amazonaws.com/dropzone/with_lxml.xlsx

These files can be unzipped and inspected for possible explanations.
