Woohoo! Sorted by Overdue, Today, Tomorrow and Someday.
Fixes bug where empty list of todo will also display.
Fixes duplicated todo in due to sorting logic.
Enhance date display
Changes to ToDoEdit.java and CategoryPriorityParser.java
- check category for "+" if dont have append into category string
- rename variable in CatgoryPriorityParser.java
Revert "Changes to ToDoEdit.java and CategoryPriorityParser.java"

This reverts commit dada6efde222502db40df485f6e275ba09e49934.
Changes to ToDoEdit.java and CategoryPriorityParser.java
- check category for "+" if dont have append into category string
- rename variable in CatgoryPriorityParser.java
add comments to test case files
Patched reverted commit from @neoengtai
Added `upcoming` todo :)
New sorting algorithm without priority.
Cater for addition of empty task
Priority sorting.
Fixes not ignoringCase for title during compare.
Throws exception for invalid id given.
More Test Cases for edit function
Merge pull request #124 from cs2103aug2014-t17-3j/Sorting

Woohoo! Sorted by Overdue, Today, Tomorrow and Someday.