added an item and inventory system

- an item class has been added that functions much like the Entity class in that:
   * it extends the Glyph class so that items may have an appearance
   * is defined by a template which lists an items properties
- item templates are defined in items. js
   * each item must have a use() function, which defines its behavior. The function takes a user and target parameter
   * I resisted the tempation to use mixins for items with similar effects (healing items, damage items, etc),
     as it seemed like over-engineering at this stage.
- items are managed by an inventory class, which keeps track of items and quantities
   * items are added using addItem(), where items are given a letter (for using) and pushed into the array
   * because of the way that Inventory was designed, even monster inventories will be given letters, though
     they won't be used
Merge branch 'items' of ssh://github.com/a-out/avast_roguelike into items