Trigger的相关属性详细解读请看如下链接：
http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_triggers_context_variables.htm
详细的创建步骤请看如下链接：
http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_qs_HelloWorld.htm
http://www.salesforce.com/us/developer/docs/apexcode/index_Left.htm#StartTopic=Content/apex_triggers.htm
