tobias: modified mode_countdown.vhd
deliverable: overview of time_buffer contains fewer implementation details

moved previous overview to implementation and fixed a missing wire
Merge branch 'master' of github.com:ic-design-wise12-13/ic-design-wise12-13
mode_countdown: fix more errors

 - add forgotten assignment (remove implicit latch)
 - 60 seconds to a minute instead of 61
mode_alarm: small fixes

 - alarm should only start ringing on full minute
 - correctly handle switching the alarm on and off