Upgrade to Django 1.6.2

This commit upgrades CATMAID to use Django 1.6.2, the most recent stable
release of the Django web framework. I went through all release notes and
upgrade instructions from version 1.5 on. Most of the things are no issue
for CATMAID, but some smaller changes are required. The following things
have been changed due to the upgrade:

* The TransactionMiddleware is deprecated and replaced by the
  ATOMIC_REQUESTS field in the database configuration. This is basically
  the same, but there are two differences: 1. one cannot switch between
  commit and autocommit within a view anymore (CATMAID didn't make use
  of this) and 2. ATOMIC_REQUESTS only applies to DB access within views
  (including cursors). TransactionMiddleware applied to all middleware
  classes coming after it in the list (CATMAID didn't make use of this,
  TransactionMiddleware was last).
* Updated the manage.py code to the currently recommended format.
* Updated django-taggit to its recent version to be compatible with
  Django 1.6. This application started using South. Therefore, it's
  first migration has to be faked (see below).
* Update django-guardion to version 1.1.1, because it didn't support
  Django 1.6 before.
* Update django-adminplus to version 0.2.1, because it didn't support
  Django 1.6 before.
* Update django-devserver to git version after 0.7.0, because it caused
  an infinite recursion in Django 1.6 before (needs to be replaced as
  soon as new version is out:
  https://code.djangoproject.com/ticket/21348
* Update South to version 0.8.4, because it didn't fully support Django
  1.6 before.
* Some Django namespaces have changed and required some imports to change.

To migrate the database, you have to do the following:

  1. Activate virtualenv
  2. Update virtualenv: pip install -r pip-frozen
  3. manage.py syncdb
  4. manage.py migrate taggit --fake 0001
  5. manage.py migrate

Your settings.py file needs a new setting called ALLOWED_HOSTS. Please
add it manually or re-run the create_configuration.py script. It is a
list of allowed hosts that are tested against a request's Host header. More
Info can be found in the documentation [1]:

  ALLOWED_HOSTS = ['CATMAID_SERVERNAME']

If you see the error 'No module named five' when calling manage.py, uninstall
and then install again django-celery:

 pip uninstall django-celery
 pip install django-celery

References:

[1] https://docs.djangoproject.com/en/1.6/ref/settings/#std:setting-ALLOWED_HOSTS
Merge branch 'move-to-django-1.6'