Use stdlib csv module to write csv
I noticed while using pandas that data with commas in the fields wasn't getting escaped properly when written to CSV (data in biology often has bits of text as well as numbers). This causes the data to spread out over extra columns. This commit uses the `csv` module from the standard library to write out CSV 'properly'.

I've also added a simple test for this case.

Cool. I was actually made aware of this a while ago but as you can see forgot to fix it ;) I'm sure performance is better, too. 
 I wouldn't expect a big performance difference, but it may be a bit faster, since I think csv.writer is implemented in C.
