Re-enable localisation of flash messages after migration to Flask 0.10+

Flask 0.10+ makes it impossible to use `login_manager.login_message = lazy_gettext('Please log in')` (no more pickle), thus buggering localisation of `flask-login`'s flash messages.

Add an optional `localizer` argument/attribute (e.g. `gettext`) to `LoginManager` that, if present, can translate `login_message` and `needs_refresh_message` before they're sent to `flash`.

Needs a better name and documentation. Works, though :)