Added Comment Modifier to Mysql Grammar

Hi Taylor,

Love your work Laravel is just awesome!!

I really care about documentation and always find my adding comments in the comments column in mysql

Till now I would do something like this

```php
Schema::create('users', function($table) {
			$table->integer('extranet_id')->nullable()
		});

DB::update("ALTER TABLE `users` CHANGE `extranet_id` `extranet_id` INT(255)  COMMENT 'Comment about the field here!'");
```

This seems like double work. 

So I have created a fork and added this functionality, please review and merge if all ok.

Now you can do something like this.
```php
Schema::create('users', function($table) {
			$table->integer('extranet_id')->nullable()->comment('Comment about the field here!');
		});
```

I also created a test but cant seem to find where to put it. There is no tests folder or repo?

Here is the test

```php
	public function testAddingComment()
	{
		$blueprint = new Blueprint('users');
		$blueprint->string('foo')->comment('foo');
		$statements = $blueprint->toSql($this->getGrammar());

		$this->assertEquals(1, count($statements));
		$this->assertEquals("alter table `users` add `foo` varchar(255) not null comment 'foo'", $statements[0]);
	}
```

Lastly I will also update the documentation when you merge the change.

Sorry I didn't update the other grammars as I not familiar with their syntax.