Integration with RabbitMQ and Celery

First update taiga-vagrant vm if you're using it, you will have access
to the rabbit management console at port 8001 of the host machine.

* Defining tasks
- Tasks must be defined in a `deferred` module of an app, for example,
`taiga.projects.deferred` module.
- Tasks must be decorated and given the name "<app>.<task-name>", for
example in `taiga.projects.deferred` module:
```
from taiga.celery import app

@app.task(name="projects.add")
def add(x, y):
    return x + y
```
- Tasks should be at most just wrappers around service functions to
promote re-usability of those functions from other parts of the code,
say, management commands and the like.

* Calling tasks
Tasks should be called using one of the three functions defined in
`taiga.deferred` module:

- `defer`: Use this function if you need to perform some task
asynchronously and GET THE RESULT back later, for example:
```
result = defer("projects.add", x=1, y=2)
...
result.get() # => 3
```

- `call_async`: Use this function when you want to fire off some
task. No result is get back. For example:
```
call_async("projects.email_user", user)
```

- `apply_async`: Is the same as `call_async` but since it's a function
application and you must pass the args and kwargs together as one
parameter each you are allowed to pass celery-specific
extra-options (but bear in mind this is not recommended!)
```
apply_async("projects.email_user", args=(user,), kwargs={}, routing_key="tasks.email")
```