Update Celery to version 3.1.7 and Django-celery to version 3.1.1

The integration of Celery in Django has changed from version 3.1 on,
django-celery is not required anymore to start tasks, etc.. However, it might
still be of use for CATMAID, since it can be used for the database result
backend and the Django periodic task admin.
Remove django-kombu, because of it's inclusion in kombu [settings update]

Please update your settings files according to the modified settings.py.example:
Replace the lines

  INSTALLED_APPS += ("djkombu",)
  BROKER_BACKEND = "djkombu.transport.DatabaseTransport"

by these lines:

  INSTALLED_APPS += ("kombu.transport.django",)
  BROKER_URL = 'django://'
Connector export: add indentation style info to template
Exports: use unobtrusive JavaScript to bind to links

This makes the links work in Firefox and lets the browser not open a new tab
anymore.
Connector export: add link, description and dialog skeleton
Connector export: add basic dialog controls
Connector export: make sure only one stack is loaded in front-end
Connector export: add skeleton retrieval
Connector export: add backend infrastructure code
Connector export: specify output directory [settings update]

A new settings variable MEDIA_CONNECTOR_SUBDIRECTORY is introduced to specify a
subdirectory of MEDIA_ROOT in which created connector archives are stored. The
back-end checks if this folder is accessible and returns an error if not.
Move random ID generator to common module
Connector export: add connector retrieval and output infrastructure

This commit adds the retrieval of many or single connectors (if a sample is
wanted), the creation of an output folder with a random name and based on this a
tar.gz file. Nothing is added to the output folder/tar file, yet.
Connector export: create connector folder structure

This commit adds code so that the folder structure for every exported
connector is created. This folder structure is:

  output_path/neuron_id/[presynaptic|postsynaptic]/connector_id

In case a connector is not linked with the presynaptic_to or the
postsynaptic_to relation of the current project, a folder named
'unknown_<relation_id>' is created instead.
Cropping: make it possible to disable message creation

Not all modules that are using the cropping functionality want to have a message
created.
Cropping: make message creation code more readable
Cropping: add infrastructure to add more tile source types
Cropping: add support for tyle source type 4
Cropping: return error from main handler if one happened
Connector export: use more precise naming
Cropping: remove usage of Hop-by-hop 'Connection' header

According to PEP 333 (WSGI), applications are not allowed to use such headers:

  Applications and middleware are forbidden from using HTTP/1.1 "hop-by-hop"
  features or headers, any equivalent features in HTTP/1.0, or any headers that
  would affect the persistence of the client's connection to the web server.

  (http://www.python.org/dev/peps/pep-0333/)

If in use, some WSGI servers raise errors (i.e. django-devserver).