Implement undo. New library added: cloning-1.8.5 for creating copies of objects. Undo display/search is not supported
remove redundant implement in task
And class path. To do: write unit tests
Added various unit tests
Added test for doing commands without default fields
Merge branch 'pn-logic-undo' of https://github.com/cs2103aug2014-w13-4j/main
Merge branch 'pn-logic-undo' of https://github.com/cs2103aug2014-w13-4j/main