Issue #2: Drag and Drop Events

- Drag and update events
- Dragged event start/end times are updated
- Overlapping events during the drag are squeezed
- Prevent dropping events over completed events

- Add draggabilly as a dependency
- Events are trigged for squeezed events
- Drag handlers to update event
- Add Event.changeStart method to change start and have end increase by
  duration
- EventCollection.getNow should only compare the minute