Added support for alter table with unique constraint syntax in MySQL.

[uehara-delta]

This adds support for alter table with unique constraint syntax in MySQL.

For example,

create table `example` (
  id integer not null,
  `subid` varchar(255),
  `subid2` varchar(255),
  primary key (`id`)
);

alter table `example`
  add constraint subid_uniq unique key (`subid`, `subid2`);