Support different delimiters for to_csv
This pull requests implements delimter support into to_csv. A number of analysis tools expects TSV files rather than CSV files, and this is a simple solution to the problem.

The delimiter parameter is by default ",", hence there are no side effects with existing code.

thanks! merged it into master
 i'm going to change the argument name to `sep` instead of delimiter to conform with `read_csv` and `read_table`
