Updated Delete Item Functionality.

- Delete item method now takes an item ID instead of an index. Other item methods will adopt this as well, will be more reliable than hoping an index points to the correct item.
- Some minor code style changes.
Changed Model Methods in Angular Controllers to Use IDs Instead of Indexes.

Indexes are used behind the scenes via controller methods, but constantly changing indexes in the view would cause problems sometimes when used as method parameters (e.g., the wrong model being modified). Passing model IDs is at least consistent.