should display 0 instead of empty string.

add doUnaryOperator() function

use Math.pow() instead.

added doc on how to add new operator button.

- implemented undo using snapshots.
  I kept recent 1000 snapshots in a bounded stack.
  note: snapshot must clone the underlying array instead of shadow copying.
- bugfix: when input invalid number like "2.3.", the error msg was not
  shown on #error-msg.
- use monospace font on #number-display

add function in fsm and add relevant buttons on UI.

show scrollbar on stack and trait when content overflows.

- feature: backspace should delete number in stack in idle mode
- add error handling for "division by 0"
- bugfix on isNumberKey(), it's more strict now
- show error msg in web UI

added unit test for it.