Implement new delete API

https://github.com/cs2103aug2014-f11-3j/main/issues/67

Why is this change necessary?
The delete method for database has changed (see link).

How does it address the issue?
- Deletion is now done by task ID.
- Appropriate exceptions are thrown.

What side effects does this change have?
Callers of this delete method, e.g. Logic, have to handle exceptions
thrown from this delete method appropriately.