Wrote an undo method for Add
Added an undo method for Edit
Added an undo method for remove
Currently editing undo
Removed unnecessary variables and methods
Uncommented the push for commands into the Undo stack
Merge branch 'master' of https://github.com/cs2103aug2014-t7-2/main

# By Natalie Rawle
# Via Natalie Rawle
* 'master' of https://github.com/cs2103aug2014-t7-2/main:
  Made undo methods
  added trivial undo methods