Adding a --no-inference option to in2csv in order to disable type inference

I needed this patch in order to use csvkit with csv files containing phone numbers (I don't want them to be interpreted as integers and have the + or the leading zeros removed) or "None" strings (yes, there's a small town in Italy called None :P)

After the possible merge you should provide a --no-inference option to the other scripts as well (i wrote only the one for in2csv as I just needed it).

Thanks :)