Allow connexion without user_searchbase

[anonymous]

My Novell LDAP server doesn't keep user list into an 'ou', so I had to leave user_searchbase, but then the LDAP searchbase is invalide because it starts with a comma (,).

Here's a proposed patch (my knowledge of Python are quite basic) :

LINE 13 : The default user_searchbase is now empty

user_searchbase = Option('ldap', 'user_searchbase', '')

LINE 24 : Strip separators (comma) at string start and end

base = (self.user_searchbase + ',' + self.root_dn).strip(',')