More clever work fetch

[saldsl]

At the moment BOINC only takes account for % of time PC is on and % of BOINC is running to calculate how much work to fetch and when. The process could be more clever, taking care of which days of week PC/BOINC is running and which not. For example, an office PC will stay on from Monday to Friday and off Saturday and Sunday, so BOINC should know that if today it's Friday for the next two days no work will be done. Moreover, it can be implemented a sort of calendar where the user can insert, for example, holidays, to tell BOINC that for some day or some week the PC will remain off, so it should not download any more WUs. (currently the workaround I'm using it's suspend work fetch for projects with short deadlines)