use Math.pow() instead.

keyboard is useful on the desktop.
TAB and backspace is not supported yet.

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

all basic things should be working except "undo" feature.

should be easy to use on portrait mode.
landscape mode may require additional work to be pleasant.

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.