Introducing the ``aggregate`` kwarg to ``_get_queryset_relatives``

**Explanation**:


I'll use the following tree from [this article](http://www.sitepoint.com/hierarchical-data-database-2/) in the explanation that follows:

![mptt tree](http://sitepointstatic.com/graphics/sitepoint_numbering.gif)


Given the tree above, we know that in order to retrieve the descendants of the ``Meat`` node, we need to execute an SQL query that looks like this:

	SELECT * FROM tree WHERE lft BETWEEN 12 AND 17;

And for the ancestors of the ``Pork`` node, we'd need to execute an SQL query that looks like this:

	SELECT * FROM tree WHERE lft < 15 AND rght > 16;

But what if we wanted the all of the descendants of both the ``Fruit`` and ``Meat`` nodes? We could do the following:

	SELECT * FROM tree WHERE lft BETWEEN 2 AND 11 OR lft BETWEEN 12 AND 17;

And if we wanted to retrieve all of the ancestors of the ``Red``, ``Beef`` and ``Pork`` nodes, we could do this:

	SELECT * FROM tree WHERE lft < 3 AND rght > 6 OR lft < 13 AND rght > 14 OR lft < 15 AND rght > 16;

Now take a look at that last query again. Couldn't it be simplified and still accomplish the same result? Sure it could:

	SELECT * FROM tree WHERE lft < 3 AND rght > 6 OR lft < 15 AND rght > 14;

The difference is that we've put the nodes into groups and then identified the highest ``lft`` value and the lowest ``rght`` value per group. A group is defined as any collection of nodes that share a ``tree_id`` and a ``parent_id``. Any ancestors of these groups of nodes will have a ``lft`` value less than the highest ``lft`` value and a ``rght`` value greater than the lowest ``rght`` value. 

For the descendants of a group of nodes, it is just the opposite: Any descendants of a group of nodes will have a ``lft`` value greater than the lowest ``lft`` value and a ``rght`` value less than the highest ``rght`` value. For example, if we wanted to retrieve the descendants of ``Red``, ``Yellow``, and ``Meat``:


	SELECT * FROM tree WHERE lft > 3 AND rght < 10 OR lft > 12 AND rght < 17;


**Why?**

Both ``get_queryset_descendants`` and ``get_queryset_ancestors`` use a ``Q`` object in order to accomplish their job: retrieving either the ancestors or descendants of a queryset (i.e., group) of nodes. This ``Q`` object can become quite large and in some cases it can raise an ``OperationalError`` if there are too many SQL variables involved. For example, ``sqlite`` only allows a single query to have <1000 SQL variables; well, if you wanted to get the ancestors of a queryset that contains 400 distinct nodes, you're looking at a query that contains 1200 variables. Now we could discuss the various techniques involved in shrinking that queryset, but with sufficiently large sets of data, sometimes there just isn't a way to do it.

That's where the ``aggregate`` kwarg comes into play. This kwarg tells ``get_queryset_ancestors`` and ``get_queryset_descendants`` to group nodes first by their ``tree_id`` then by their ``parent_id``. Then, for each "group", it finds the the appropriate ``lft`` and ``rght`` values to ensure that there is only one ``Q`` object per group of nodes. To reuse the example above, if you had a queryset with 400 distinct nodes but 200 of them shared the same ``tree_id`` and ``parent_id`` and the other 200 shared a different ``tree_id``/``parent_id`` combination, you would have two groups, with a total of 6 SQL variables (``tree_id``, ``lft``, ``rght`` variables for each group.) Not only does this make for faster queries, but it has the potential to avoid the scenario I mentioned earlier where too many SQL variables raises an ``OperationalError``.


**Examples:**

``models.py``

    from django.db import models
    from mptt.models import TreeForeignKey, MPTTModel
    
    class Directory(MPTTModel):
        parent = TreeForeignKey(
            'self',
            null = True,
            blank = True,
            related_name = 'children')
    
        path = models.CharField(
            max_length = 255)
    
        def __unicode__(self):
            return self.path
    
    
    class File(models.Model):
        parent = models.ForeignKey(
            Directory,
            null = True,
            blank = True,
            related_name = 'files')
    
        path = models.CharField(
            max_length = 255)
    
        def __unicode__(self):
            return self.path


Our dataset is a directory structure that matches the example tree in the above graphic:
	
	mkdir -p Food/Fruit/Red/Cherry
	mkdir -p Food/Fruit/Yellow/Banana
	mkdir -p Food/Meat/{Beef,Pork}

Then:

	import os
	from myapp.filesanddirectories.models import *

	for root, directories, files in os.walk('/Food'):
    	parent_dir, created = Directory.objects.get_or_create(path = root)
	    for directory in directories:
        	path = os.path.join(root, directory)
	        print path
        	object, created = Directory.objects.get_or_create(path = path, parent = parent_dir)
	        if created:
        	    print("CREATED: %s" % path)


Running in the python interpreter (with debugging output):

	>>> from animalsapp.filesanddirectories.models import *
	>>> from django.db.models import Q
	>>> from django.db import transaction
	>>> dirs = Directory.objects.filter(Q(path = '/Food/Fruit') | Q(path = '/Food/Meat'))
	>>> dirs
	[<Directory: /Food/Fruit>, <Directory: /Food/Meat>]
	>>> 
	>>> ### get_queryset_descendants ###
	>>> 
	>>> transaction.commit()
	>>> Directory.objects.get_queryset_descendants(dirs)
	Finished in 0.000601
	Q Object: (OR: (AND: (u'lft__gt', 2), (u'tree_id', 1), (u'rght__lt', 11)), (AND: (u'lft__gt', 12), (u'tree_id', 1), (u'rght__lt', 17)))
	[<Directory: /Food/Fruit/Red>, <Directory: /Food/Fruit/Red/Cherry>, <Directory: /Food/Fruit/Yellow>, <Directory: /Food/Fruit/Yellow/Banana>, <Directory: /Food/Meat/Pork>, <Directory: /Food/Meat/Beef>]
	>>> 
	>>> transaction.commit()
	>>> Directory.objects.get_queryset_descendants(dirs, aggregate=True)
	Finished in 0.000036
	Q Object: (AND: (u'lft__gt', 2), (u'tree_id', 1), (u'rght__lt', 17))
	[<Directory: /Food/Fruit/Red>, <Directory: /Food/Fruit/Red/Cherry>, <Directory: /Food/Fruit/Yellow>, <Directory: /Food/Fruit/Yellow/Banana>, <Directory: /Food/Meat/Pork>, <Directory: /Food/Meat/Beef>]
	>>> 
	>>> transaction.commit()
	>>> Directory.objects.get_queryset_descendants(dirs, include_self=True)
	Finished in 0.000043
	Q Object: (OR: (AND: (u'lft__gt', 1), (u'tree_id', 1), (u'rght__lt', 12)), (AND: (u'lft__gt', 11), (u'tree_id', 1), (u'rght__lt', 18)))
	[<Directory: /Food/Fruit>, <Directory: /Food/Fruit/Red>, <Directory: /Food/Fruit/Red/Cherry>, <Directory: /Food/Fruit/Yellow>, <Directory: /Food/Fruit/Yellow/Banana>, <Directory: /Food/Meat>, <Directory: /Food/Meat/Pork>, <Directory: /Food/Meat/Beef>]
	>>> 
	>>> transaction.commit()
	>>> Directory.objects.get_queryset_descendants(dirs, include_self=True, aggregate=True)
	Finished in 0.000027
	Q Object: (AND: (u'lft__gt', 1), (u'tree_id', 1), (u'rght__lt', 18))
	[<Directory: /Food/Fruit>, <Directory: /Food/Fruit/Red>, <Directory: /Food/Fruit/Red/Cherry>, <Directory: /Food/Fruit/Yellow>, <Directory: /Food/Fruit/Yellow/Banana>, <Directory: /Food/Meat>, <Directory: /Food/Meat/Pork>, <Directory: /Food/Meat/Beef>]
	>>> 
	>>> 
	>>> ### get_queryset_ancestors ###
	>>> 
	>>> transaction.commit()
	>>> Directory.objects.get_queryset_ancestors(dirs)
	Finished in 0.000042
	Q Object: (OR: (AND: (u'rght__gt', 11), (u'lft__lt', 2), (u'tree_id', 1)), (AND: (u'rght__gt', 17), (u'lft__lt', 12), (u'tree_id', 1)))
	[<Directory: /Food>]
	>>> 
	>>> transaction.commit()
	>>> Directory.objects.get_queryset_ancestors(dirs, aggregate=True)
	Finished in 0.000027
	Q Object: (AND: (u'rght__gt', 11), (u'lft__lt', 12), (u'tree_id', 1))
	[<Directory: /Food>]
	>>> 
	>>> transaction.commit()
	>>> Directory.objects.get_queryset_ancestors(dirs, include_self=True)
	Finished in 0.000045
	Q Object: (OR: (AND: (u'rght__gt', 10), (u'lft__lt', 3), (u'tree_id', 1)), (AND: (u'rght__gt', 16), (u'lft__lt', 13), (u'tree_id', 1)))
	[<Directory: /Food>, <Directory: /Food/Fruit>, <Directory: /Food/Meat>]
	>>> 
	>>> transaction.commit()
	>>> Directory.objects.get_queryset_ancestors(dirs, include_self=True, aggregate=True)
	Finished in 0.000091
	Q Object: (AND: (u'rght__gt', 10), (u'lft__lt', 13), (u'tree_id', 1))
	[<Directory: /Food>, <Directory: /Food/Fruit>, <Directory: /Food/Meat>]
	>>> 

Summary
=======

Please tear this to shreds. Seriously. I haven't been able to find any bugs with this implementation but that doesn't mean that they aren't there. Tests pass though so ... yay. :-)