pg8000 dialect now supports client_encoding from create_engine()

Hi Mike, a couple of commits for the pg8000 dialect.

The first one makes the checking of the \__version\__ attribute cope with the version string you get in development when you use [Versioneer](https://github.com/warner/python-versioneer), as pg8000 does.

The second commit adds support for setting the client_encoding parameter from create_engine, in the same way as with psycopg2.