Add index and header options to DataFrame.to_string()
Request from mailing list [discussion](https://groups.google.com/d/topic/pystatsmodels/XPuDEfFKU8U/discussion).

`DataFrame.to_csv()` have _index_ and _header_ argument, request is to add this functionality also to `DataFrame.to_string()`.

Maybe do this also for `DataFrame.to_html()`?
