Function to create an MIT user with LDAP data

Function to create an MIT user with LDAP data

This adds a function get_or_create_mit_user. As with the "get_or_create"
methods on managers, this returns an object satisfying some conditions,
creating it if necessary. In this case, we return a User object that's
populated using data from MIT's LDAP. If the user does not exist and
cannot be found in LDAP, we raise an exception.