Update docs to remove use_cache param
Have a default for the 'user' connect() parameter

If the connect() parameter 'user' is not provided, look first for the
PGUSER then the USER environment variables. Thanks to Alex Gaynor
https://github.com/alex for this suggestion.
Before PostgreSQL 8.2, COPY didn't give row count

Until PostgreSQL 8.2 (which includes Amazon Redshift which forked at
8.0) the COPY command didn't return a row count, but pg8000 thought it
did. That's fixed now.