Remove clean_sessions task

The clean_sessions task isn't used for anythin in openspending
any more and we strive to keep the code clean.
Split analysis and loading queues in celery

Loading csv files into OpenSpending can take a long time and block
all of the other tasks. Analysis of csv files is a lot faster and
the message given to user is (correctly) 'this should not take long'.

It doesn't take long but it can happen that it just has to wait for
a long time while some big loading tasks are running.

Splitting analysis and loading into two different queues allows us
to run separated workers for each of these tasks and therefore not
block analysis tasks with big loading tasks.

There will in effect be three queues: analysis, loading and the
default queue 'celery'. The default queue handles tasks which are
not explicitly routed to some specific queue (namely the generic
ping task which is used to check if celery is working and not
really needed in production).
Update documentation based on new celery queue setup

Workers must be started to run and use the three new celery queues
Bump to version 0.14

This is a small version bump but still required since the celery
app now uses three different queues to better distribute tasks
between workers and reduce blocking of task processing.

This is not backwards compatible since the celery workers must
now be started with the -Q option do define the queues.
Merge pull request #808 from openspending/separate-celery-queues

Separate celery queues