Add DateTime support to database binding layer.

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

laravel/database/query/grammars/grammar.php

@@ -5,6 +5,13 @@ use Laravel\Database\Expression;
 
 class Grammar extends \Laravel\Database\Grammar {
 
+	/**
+	 * The format for properly saving a DateTime.
+	 *
+	 * @var string
+	 */
+	public $datetime = 'Y-m-d H:i:s';
+
 	/**
 	 * All of the query componenets in the order they should be built.
 	 *