Added $setAsDefault - if value is undefined on update, set the given value
 Added $limit, $skip and $sort aggregation options
aggregation is always ON along with the SORT option, while limit/skip will be used only on MULTI:true.
aggregation is always ON along with the SORT option, while limit/skip will be used only on MULTI:true.

Add aggregate to find/findOne
Merge http://github.com/Ivshti/nedb