Ticket #554 (assigned)

add the possibility of having 'notes' in the scheduler. a note has to following properties: - start time - end time - title (text) - contents (text) notes with themselves may not overlap, but notes and scheduled entries may overlap. (thus, it's like having two layers: a notes layer, which is consitent on its own, and a scheduled entry layer on top - with the two layers overlapping in any possible way) other than the above, notes don't trigger any event in the schedule - they are just placeholders of information. and API is needed to: - add a note - edit / update a note (move times, update content, etc) - remove a note - get notes for a given time frame - get the next note for a given time