Added UNSIGNED to mysql grammar inrementer()

Added UNSIGNED to the mysql grammar file because auto increment fields
only run upwards, it's a waste of space to provide negative values by
the default SIGNED state of INT.

laravel/database/schema/grammars/mysql.php

@@ -143,7 +143,7 @@ protected function incrementer(Table $table, Fluent $column)
 	{
 		if ($column->type == 'integer' and $column->increment)
 		{
-			return ' AUTO_INCREMENT PRIMARY KEY';
+			return ' UNSIGNED AUTO_INCREMENT PRIMARY KEY';
 		}
 	}