Else statement needed..and USERNAME dependency. #57

Hi!

There is a use case not being handled here, causing it to fail on my WinXP machine(variable isn't initialised).
It is at line 142 at clientparameters.py:

if sys.platform == 'win32':
    appdata = environ.get('APPDATA')
    if appdata:
        pgdata = os.path.join(appdata, pg_appdata_directory)
        pgpassfile = os.path.join(pgdata, pg_appdata_passfile)
else:

Adding an else statement on appdata like at least makes the error go away and the test succeed on my install:

    if appdata:
        pgdata = os.path.join(appdata, pg_appdata_directory)
        pgpassfile = os.path.join(pgdata, pg_appdata_passfile)
    else:
        pgpassfile = ''

Also, the USERNAME environment variable seem to be necessary on windows but not on linux.
I don't see why "user" doesn't seem to be passed from postgresql.driver.dbapi20.connect?
I makes it far less portable.

Otherwise, great work!