Support scale in MainClock

- if the mScale is set, the angle will always be rounded to the nearest scale
Add digital clock in the SettingActivity

- add 4 textviews in the layout
- register a BroadcastReceiver in the `onCreate()` to listen on the time ticks
- update the digital clock according to the system time
connect MainClock with updating digital clock

- in order to trigger the `updateDigitalClock()`, dragging listener
  is added.
- add a `getTime()` method to return the value of MainClock:
    - add a member `mScaledValue` in `Hand` to store the value in
      scale;
    - also fix a previous bug: the angle calculation in `Hand` is
      wrong, the direction of axis is messed
- enhance the `updateDigitalClock()` in `SettingActivity` to jduge
  whether to display system time or the time from `mainClock`