added a tiny utility: show_item_id.rb

The utility shows Item ID form the header file (ItemEnum.h) that
contains a enum block.

<< Example to use>>

$ ./show_item_id.rb ../../src/ItemEnum.h
----: /* Asura
----:    Copyright (C) 2013 MIRACLE LINUX CORPORATION
----:
----:    This program is free software: you can redistribute it and/or modify
<snip>
----: enum
----: {
0000:   ITEM_ID_NOT_SET,
0001:   ITEM_ID_NOBODY,
----:
0002:   ITEM_ID_ZBX_NODES_NODEID,
0003:   ITEM_ID_ZBX_NODES_NAME,
0004:   ITEM_ID_ZBX_NODES_IP,