add in test for conditional task
fix conditional date pair problem
tree map refactor
minor changes for tag storage
modify filter functions
Merge branch 'chuyu-storage' of github.com:cs2103aug2014-w13-4j/main into pn-logic-filter

Conflicts:
	src/models/Task.java
	src/tests/TaskStorageTest.java
Merge branch 'master' of github.com:cs2103aug2014-w13-4j/main into pn-logic-filter
Add in filter for completed and active task. Command is filter status <completed/active>
Also added check that mandatory fields are present in adding/deleting tasks