Added operations

I have added some functions to NSManagedObject:

1. Count
2. Sum
3. Max
4. Min
5. Average

Count 
```swift
class func count(context: NSManagedObjectContext = SuperCoreDataStack.defaultStack.managedObjectContext!, predicate : NSPredicate?, error: NSErrorPointer) -> Int {
            var entityName : NSString = NSStringFromClass(self)
            var fetchRequest = NSFetchRequest(entityName: entityName);
            fetchRequest.includesPropertyValues = false
            fetchRequest.includesSubentities = false
            fetchRequest.predicate = predicate
            fetchRequest.propertiesToFetch = [];
            return context.countForFetchRequest(fetchRequest, error: error)
    }

```
is more correct and efficient than

```swift
class func countWithPredicate(predicate: NSPredicate!, context: NSManagedObjectContext = SuperCoreDataStack.defaultStack.managedObjectContext!, completionHandler handler: ((NSError!) -> Void)! = nil) -> Int {
            let results = findAllWithPredicate(predicate, context: context, completionHandler: handler)
            return results.count
}
```

All other functions use NSExpression to compute sum, min, avg, max, min of one ore more field of the NSManagedObject