added comment property support in postgres dialect

[NAlexandrov]

If you open the url http://docs.sequelizejs.com/en/latest/docs/models-definition/ you can see that:

// Comments can be specified for each field for MySQL and PG
hasComment: { type: Sequelize.INTEGER, comment: "I'm a comment!" },

But this feature was not implemented in postgres dialect. I added this.

[NAlexandrov]

Errors in travis-ci not related with my commits. And I can't fix it. I close this PR. May be someone else will add support a comment property for each field.

[mickhansen]

Build looks green?

[janmeier]

I restarted it :)

[mwain]

Just came across this, i created a pull (#4767) for the same functionality and didn't see your pull.
While testing this it doesn't work for me, attribute.Model is undefined when using queryInterface. E.g:

queryInterface.createTable(
    'test',
    {
        id: {
            type: Sequelize.INTEGER,
            autoIncrement: true,
            primaryKey: true,
            comment: 'Unique identifier'
        }
    },
    {
        schema: 'playground'
    }
);

Also, will it handle schemas?