Merge branch 'master' into logic/wenhao
Add "delete all"
Fix bug with Undo. Issue #61
Merge remote-tracking branch 'origin/ui/victor' into logic/wenhao
Changes in method flow, variable names
Merge branch 'master' into controller/sharon
A lot of changes
Merge remote-tracking branch 'origin/ui/victor' into controller/sharon
Complete HelpHandler
Merge remote-tracking branch 'origin/parser/Victor' into controller/sharon
Merge remote-tracking branch 'origin/ui/victor' into controller/sharon
Major changes to controller flow

Checked for valid command type in resulting command and result classes returned by function calls to other components.
Organization and refactoring of code.
No longer uses Exception as program control flow.
[error] Still have errors, waiting for other components.
Merge remote-tracking branch 'origin/controller/sharon' into logic/wenhao
Merge remote-tracking branch 'origin/parser/Victor' into logic/wenhao
Undo return a result now.