Simplified WriteableCollection.delete()

* Renamed PLS (Promise-local Storage) to PSD (Promise Specific Data)
* Allowing function(){delete this.value;} in
WriteableCollection.modify()
* Implementing WriteableCollection.delete() via
this.modify(function(){delete this.value;});
* Changing the resolve value of delete() from array of deleted records
to just the count of deletions made (as with modify()).