Django-ddp

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* Add `dddp` console script to start DDP service in more robust manner than using the dddp Django mangement command.

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* Don't include null/None reply from method calls in message.
* Force creation of Alea/Meteor ID even if nobody seems to care -- they
do care if they're using the ID with latency compensated views.
* Support collections to models having non-integer primary key fields.
* Fix latency compensated Alea/Meteor ID generation to match Meteor
semantics of using a namespace to generate seeded Alea PRNGs.

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* Change validation so that we now pass the DDP test suite
<http://ddptest.meteor.com/>.
* Add lots of useful info to the README.

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* Add `dddp.models.get_meteor_id` and `dddp.models.get_object_id`
methods.
* Add `Connection`, `Subscription` and `SubscriptionColleciton` models,
instances of which are managed during life cycle of connections and
subscriptions.
* Fixed incorrect use of `django.core.serializers` where different
threads used same the serializer instance.
* Add `Collection.user_rel` class attribute allowing user-specific
filtering of objects at the collection level.
* Add `dddp.test` test project with example meteor-todos/django-ddp
project.
* Change `dddp` management command default port from 3000 to 8000.
* Validate `django.conf.settings.DATABASES` configuration on start.
* React to `django.db.models.signals.m2m_changed` model changes for
ManyToManyField.
* Add dependency on `django-dbarray`.

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* Add missing dependencies on `gevent`, `gevent-websocket`,
`meteor-ejson` and `psycogreen`.
* Meteor compatible latency compensation using Alea PRNG.
* Add `dddp.THREAD_LOCAL` with factories.
* Register django signals handlers via `AppConfig.ready()` handler.
* Add `dddp` management command.
* Add `dddp.models.AleaIdField` and `dddp.models.ObjectMapping` model.
* Major internal refactoring.

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* Working proof-of-concept.