Improved Project Architecture & Item System

- Auto weapon polygon collider
> weapon collider will now auto reshape every time a new weapon is
equipped.

- Refine Item Database
> store all itemicons with associated itemnames in a dictionary
> Item Database now hold this dictionary with all itemicon information.
> serialized Item, Item Database and relevant classes so that non
programmers can add new items to item database by simply working in
inspector window.

- Runtime loading assets
>  now sprite sheet can be loaded at run time, and sub sprites are
accessible as well.

- Refine project architecture
> Redesign several classes using Singleton Pattern