improving the speed of  to_csv 
Hello,

I dont know if that is possible, but it would great to find a way to speed up the `to_csv` method in Pandas. 

In my admittedly large dataframe with 20 million observations and 50 variables, it takes **literally hours** to export the data to a csv file. 

Reading the `csv` in Pandas is much faster though. I wonder what is the bottleneck here and what can be done to improve the data transfer.

Csv files are ubiquitous, and a great way to share data (without being too nerdy with `hdf5`and other subtleties). What do you think?
