Added some official item packages!
- Added db/re/item_package.txt, imported file for item_group_db.txt
- Added doc/item_group.txt for more details about item group
- Added new IG_ids on db/const.txt, up to 300 additions
- Added 'getgroupitem' script command to get item from specified item group (see doc/script_commands.txt)
- Modified 'grouprandomitem', added an optional parameter: 'sub_group' (see doc/script_commands.txt)
- Modified 'getrandgroupitem', added an optional parameter: 'sub_group' (see doc/script_commands.txt)
- Updated db/re/item_db.txt
- Updated sql-files/item_db_re.sql, so for MySQL user, please udate your item db
NOTE:
- I ocmmented some lines because some items aren't exist yet on rAthena db (or maybe have different name?)
- For pre-re, maybe I can do it later, or will someone do it?
- If there's something I forget to tell you, it's habit

Signed-off-by: Cydh Ramdh <house.bad@gmail.com>