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).
Connector export: create cropping job
Cropping: respect a stack's translation