Implement all log-related methods

https://github.com/cs2103aug2014-f11-3j/main/issues/70
https://github.com/cs2103aug2014-f11-3j/main/issues/57

Why is this change necessary?
To retain tasks in task log for task retrieval after user has exited
program.

Close #57
Close #70
Merge branch 'master' of https://github.com/cs2103aug2014-f11-3j/main