code: function (columns, opts) {
              opts = opts || {};
              this.operations.push(merge({op:"addConstraint", type:"unique", columns:toArray(columns)}, opts));
          }
docs: Add a unique constraint to the given column(s).
  See {@link patio.SchemaGenerator#constraint}.
  @example
  DB.alterTable("test", function(){
  this.addUniqueConstraint("name");
  //=> ADD UNIQUE (name)
  this.addUniqueConstraint("name", {name : "uniqueName});
  //=> ADD CONSTRAINT uniqueName UNIQUE (name)
  });