BUG: read_excel doesn't respect string data
From [SO](http://stackoverflow.com/questions/33137686/python-loading-zip-codes-into-a-dataframe-as-strings/33137797#33137797)

``` python
In [16]: df = pd.DataFrame({'a': ['001','002']})

In [17]: df.to_excel('temp.xlsx')

In [18]: pd.read_excel('temp.xlsx')
Out[18]: 
   a
0  1
1  2
```

I think it would probably make sense for `read_excel` to not try and convert strings to numeric, or at least have another keyword argument.
