Add support to column comments in the Postgres Adapter

Add support to column comments in the Postgres Adapter. Closes #131

Add column comment on create table:
``` php
$this->table('table1')
     ->addColumn('column1', 'string', array('comment' => 'Comments from "column1"'))
     ->save();
// expected separated query:
// COMMENT ON table1.column1 IS 'Comments from "column1"'
```

Add column comment on change column:
``` php
$this->table('table1')
     ->changeColumn('column1', 'string', array('comment' => 'Comments from "column1"'))
     ->save();
// expected separated query:
// COMMENT ON table1.column1 IS 'Comments from "column1"'
```

To remove column comment:
``` php
$this->table('table1')
     ->changeColumn('column1', 'string', array('comment' => 'null'))
     ->save();
// expected separated query:
// COMMENT ON table1.column1 IS NULL
```