Added prototype of Item System

This pull request includes a very basic implementation of the item system, mostly for a proof of concept. Here are the updates:

* Implemented a very bare-bones Item Inventory system. I created a module which will hold all of our items, along with their properties. The system has the following features:
* [1] Ability to define new items and their stats in a property. Right now it just gives atk. In the near future, weapons will give STR, which will lead to a derived stat in the player called "Total attack = stat.atk + weapon.str". I will also add more stats for weapons, including weight, hit accuracy and so on, along with derived stats for the players. For now, we just have a proof of concept.
* [2] Ability to equip an item is functional. By equipping the item in the slot, the player will gain the atk stat from the weapon. 

Currently has the following bug(s):

* [1] Player can infinitely equip an item, therefore increasing their base attack by that amount. I need to come up with a way to prevent that; if a unit equips two different items in a row, they gain attack from both items. Not good. 

[EDIT]: I actually just figured out how to fix the above bug. It will be fixed once I separate STR of a weapon and ATK of the player into different stats and put them into a derived stat. 

* [2] Right now I've hard-coded the position of the items, i.e: Silver sword is Item1 in the Item Catalog, but it is also Item(1) inside of the hero's item list. This will be touched upon sometime soon.

Future Updates
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* [1] Full set of Items. 
* [2] Can start differentiating between equip-able and usable items.
* [3] Can start derived stats for the base_player. 
* [4] Shops can be created afterwards.
* [5] Ability for Item Quantities/ Weapon Durability will be opened.
* [6] Enemy item shops will be a possibility as well.