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.
this doesn't work.  pg8000 doesn't have a "set_client_encoding()" method ?

	>>> import pg8000
	>>> pg8000.__version__
	'1.10.1-1-gdda4fc9'
	>>> conn = pg8000.connect(user='scott', password='tiger', database='test')
	>>> conn.set_client_encoding('utf-8')
	Traceback (most recent call last):
	  File "<stdin>", line 1, in <module>
	AttributeError: 'Connection' object has no attribute 'set_client_encoding'