Wrote a method for isEqual for Task
Changed Add to only create a subcommand for end date if there is a deadline
Changed ExactMatchSearcher's isTaskDuplicate to re-use code from Tasks's isEqual method
Merge branch 'master' of https://github.com/cs2103aug2014-t7-2/main

# By Natalie Rawle (2) and Yui Wei (2)
# Via Natalie Rawle
* 'master' of https://github.com/cs2103aug2014-t7-2/main:
  Added throws declarations to get the build going again.
  fixed task to handle overdue
  Fixed Date to throw an Exception if the date is invalid instead of just outputting (0,0)
  check if task has valid start and end (doesn't end before start)