This patch adds an LDAPBackend class to accounts/backends.py,
to allow Review Board to get its authentication information
from an LDAP server.

It requires the python-ldap package.

Usage: 

LDAP_URI = 'ldap://ldap.example.com:389
LDAP_UID_MASK = 'uid=%s,ou=users,dc=example,dc=com'
LDAP_ANON_BIND_UID = ''
LDAP_ANON_BIND_PASSWD = ''
LDAP_EMAIL_DOMAIN = 'example.com'
AUTHENTICATION_BACKENDS = ('accounts.backends.LDAPBackend', 'django.contrib.auth.backends.ModelBackend') 

It works locally for me; users Review Board hasn't seen before
are successfully authenticated against LDAP.