Modify task model: added completed field and a method to add tags(without duplicates)
Added a test for adding new tags
Class for parsing dates
Added in comment, and extracted a constant out
extracted more constants
remove methods to check whether command contains the param needed
Merge branch 'master' of github.com:cs2103aug2014-w13-4j/main into pn-chuyu-logic-storage-merge

Conflicts:
	src/models/Task.java