就想简单的使用Keychain，不需要Keychain Access Group ，也不需要其他复杂的应用，
就想简单的保存个用户名和密码之类的，
就想像用NSUserDefaults 那样来存储数据。
Methods as follows:
+(BOOL)setObject:(id)object forKey:(NSString *)key;
+(id)objectForKey:(NSString *)key;
+(BOOL)deleteItmeWith:(NSString*)key;
Set Item:
[SimpleKeychain setObject:@"value" forKey:@"key"];
Get Item:
[SimpleKeychain objectForKey:@"key"];
Delete Item:
[SimpleKeychain deleteItmeWith:@"key"];
