Implement task retrieval from temporary memory

Issue #72 - https://github.com/cs2103aug2014-f11-3j/main/issues/72

Why is this change necessary?
Retrieving a task consists of the following main parts:
- Retrieve task from temporary memory, e.g. searching through tasks
  stored in an arraylist.
- Temporary memory in turn retrieves from task log.

How does it address the issue?
Implementing a method that retrieves a task based on its title from
temporary memory solves the second point mentioned above.

What side effects does this change have?
Logic can use this method to retrieve temporarily stored tasks based on
their titles.

Close #72.