Updating the database

When invoked as a module:

$ python -m gb2260
usage: __main__.py [-h]
                   [--version {2012-10-31,2013-08-31,2014-10-31,2015-09-30}]
                   [--cached] [--verbose]
                   ACTION

positional arguments:
  ACTION                action to perform: either update or refresh-cache

optional arguments:
  -h, --help            show this help message and exit
  --version {2012-10-31,2013-08-31,2014-10-31,2015-09-30}
                        version to update the database with
  --cached              read the data from cached HTML, instead of the NBS
                        website
  --verbose             give verbose output

…either of update() or refresh_cache(), below, can be invoked.