Excel writing
I added a ExcelWriter class to io/parsers.py
Added a to_excel method to DataFrame and refactored to_csv to use a helper method that also serves to_excel.

I still need to add some tests, but wanted to get feedback before spending any more time.

There are still some issues, like xlwt throws exceptions when writing np.int64 types, but we should be able to special case some of that.  I also didn't add any encoding handling, although that should be reasonably easy.
