optimize to_csv if formatting isn't needed

[changhiskhan]

http://stackoverflow.com/questions/15417574/what-is-the-fastest-way-to-output-large-dataframe-into-a-csv-file

The comments complain of corrupt data. can you ask for more info (version/copy of data)?

[changhiskhan]

done

Opened PR for 0.12 pending more testing, someone please tip off that person with a comment.

addressed by #3059.