[4.2] Addition of First to Schema MySql Grammar

For MySql Laravel already supported to add a column after another column but had to my knowledge no solution to create a new column as first column in an existing table.

This pull add's support for the FIRST MySql Alter table command and will allow this:

```php
Schema::table('pancakes', function(Blueprint $table)
{
	$table->increments('id')->first();
});
```