Implements command line tool for exporting from classic.

Allows a user to retrieve their data and code from scraperwiki classic and install it locally.  User can provide the scraper short-name, and the output file name for the code and database.

The script installs a console_script called sw_classic_export which should be accessible on the path after the changes to setup.py.  The script can be run with

    sw_classic_export hospital_smr_data -c ./code/scraper -d scraperwiki.sqlite