REF: Specify chunk_size as a keyword argument in redshift upload

We've found empirically that splitting the file into multiples of node slices results in *dramatically* faster uploads. 

Also, would you be opposed to having a redshift-specific module as part of this library, or do you think it would be best to build another lib on top of db.py? I ask because we've been tinkering with the idea of building a Redshift utility library, and you already have a number of helpful queries in queries/redshift.py.