__init__(): Removed since we no longer need the coerce flag.
    Message ids and strings are now always coerced to Unicode, /if/
    the catalog specified a charset parameter.

    gettext(), ngettext(): Since the message strings are Unicodes in
    the catalog, coerce back to encoded 8-bit strings on return.

    ugettext(), ungettext(): Coerce the message ids to Unicode when
    there's no entry for the id in the catalog.

Minor code cleanups; use booleans where appropriate.