Unicode handling in `to_latex`. Needs encoding?
I can't seem to get this one to work and `to_latex` doesn't allow a user-specified encoding. I think this might need a look.

I have it in unicode, so try that way.

```
pd.DataFrame([[u'au\xdfgangen']]).to_latex('test.tex')
```

Nope. Ok, so let's encode it as a utf-8 string

```
pd.DataFrame([[u'au\xdfgangen']]).apply(lambda x : x.str.encode('utf-8')).to_latex('test.tex')
```

Nope. It looks like it's getting coerced back to unicode in formatter._to_str_columns() then tries to write it as ASCII...
