Create a unique index on the column(s) passed.

def create_index(self, columns, table_name, if_not_exists = True, unique = False, **kwargs):
    'Create a unique index on the column(s) passed.'
    index_name = simplify(table_name) + u'_' + u'_'.join(map(simplify, columns))
    if unique:
      sql = u'CREATE UNIQUE INDEX %s ON %s (%s)'
    else:
      sql = u'CREATE INDEX %s ON %s (%s)'

    first_param = u'IF NOT EXISTS ' + index_name if if_not_exists else index_name

    params = (first_param, quote(table_name), ','.join(map(quote, columns)))
    self.execute(sql % params, **kwargs)