GitLab

This makes it easier for 3rd party code to read LDAP attributes.

Fixes #4

This works around hard-coded certificate paths in the core. A validation
against a specific certificate can still be forced by defining the setting
`LDAP_CA_CERT`.

Closes #5

Otherwise we might violate the database schema, if the values from LDAP
exceed the `max_length` defined by the user model.

The `client_strategy=RESTARTABLE` causes issues with local LDAP
servers, such as ldaptor, which might be used for testing purposes.

The old STRATEGY_SYNC_RESTARTABLE has been dropped from current releases.

Fixes #2

Raising an exception depending on `settings.DEBUG` turned out to be a bad
idea, as it breaks tests. We could try to detect Django's test mode and
surpress the exception, but that might not be reliable either.

So the best thing to do in that case is to issue a warning through Python's
logging system.

The `email` attribute is being synced from LDAP anyway and a password
prompt is usually not required as LDAP users normally use a custom
authentication backend.

This fixes Python3 compatibility

Fixes #1