django-composition 0.2

`django-composition` provides the abstract way to denormalize data from your models in simple declarative way through special generic model field called `CompositionField`.

Most cases of data denormalization are pretty common so `django-composition` has several "short-cuts" fields that handles most of them.

## Base concept

`CompositionField` is django model field that provides interface to data denormalization.

`CompositionField` constructor params:
- native - Django field instance for current compostion field
- trigger - one or some numberr of triggers that handle composition.
Trigger is a dict with allowed keys:
- on - signal or list of signals that this field handles
- do - signals handler, with 3 params:
- related instance
- instance (that comes with signal send)
- concrete signal (one from `on` value)
- field_holder_getter - function that gets instance(that comes with signal send)\
as parameter and returns field holder
object (related instance)
- sender - signal sender
- sender_model - model instance or model name that send signal
- commit - flag that indicates save instance after trigger appliance or not
- commons - a trigger like field with common settings
for all given triggers
- update_method - dict for customization of update_method. Allowed params:
- initial - initial value to field before applince of method
- do - index of update trigger or trigger itself
- queryset - query set or callable(with one param - `instance` of an holder model)
that have to retun something iterable
- name - custom method name instead of `update_FOO`

## Short-cuts

Here is the list of provided short-cut fields:

- `ForeignAttributeField`

- `ChildsAggregationField`

- `AtributesAggregationField`