Add support for comments

[PeeHaa]

We should at least support comments on:

• tables
• columns
• constraints
• indexes

[PeeHaa]

For tables:

$this->createTable('comment_test', function (Table $table) {
})->addComment('This is the table comment');

$this->createTable('comment_test', function (Table $table) {
})->removeComment();

For columns:

$this->createTable('comment_test', function (Table $table) {
    $table->addColumn('id', new BigSerial())->comment('This is the comment');
});

$this->createTable('comment_test', function (Table $table) {
    $table->addColumn('id', new BigSerial())->removeComment();
});

[PeeHaa]

PR: #23