Added foreign key support to schema builder for MySQL, Postgres, and SQL Server.

Signed-off-by: Taylor Otwell <taylorotwell@gmail.com>

laravel/database/schema/table.php

@@ -108,6 +108,17 @@ class Table {
 		return $this->key(__FUNCTION__, $columns, $name);
 	}
 
+	/**
+	 * Add a foreign key constraint to the table.
+	 *
+	 * @param  string|array  $columns
+	 * @param  string        $name
+	 */
+	public function foreign($columns, $name = null)
+	{
+		return $this->key(__FUNCTION__, $columns, $name);
+	}
+
 	/**
 	 * Create a command for creating any index.
 	 *