Store max id in DataStore. Fixes #201
Save each addition instead of waiting destructor
Merge pull request #261 from evansb/id-gen

Store max id in DataStore. Fixes #201
Merge branch 'master' of https://github.com/cs2103aug2014-w10-1c/main into implement-subtask

Conflicts:
	You-QueryEngine/internal/controller/task_graph_controller.cpp
	You-QueryEngine/internal/model/state.cpp
	You-QueryEngine/internal/model/state.h