docs: Update 0.16 upgrade notes

Describe new Pithos mapfile after the consolidation with Archipelago.
pithos: Add class attribute to ModularBackend

Add _class_version class attribute to ModularBackend class
to differentiate between multiple backend versions.
cloudbar: Do not show image link border in IE
docs: Add design doc for multi-db transactions

This doc presents the problem of multi-db transactions in Django and
proposes a solution that converges with database routers.

Also, add a mention in the dev-guide about this change.
snf_django: Add database router for Synnefo apps

Add Django database router to support using Synnefo with multiple
databases. In a gist, if the administrator has added in the DATABASES
setting a database with the "cyclades" or "astakos" alias, and
SynnefoRouter is defined in DATABASE_ROUTERS, then that database will be
used to read/write the respective Cyclades or Astakos models.
snf_django: Add support for multi-db transactions

Add two functions:

* select_db: Return an appropriate database to read/write depending on
  the app ("db"/"im") that makes the request.
* _transaction_func: Common entry point for all Cyclades/Astakos
  transaction functions. This function calls the select_db function to
  choose a database for the transaction and then passes the control to
  the Django transaction functions.
cyclades: Mimic Django transaction module

Provide a transaction module similar to Django's. The implemented
functions are commit(), rollback(), commit_on_success(),
commit_manually(), which should work exactly the same as the Django
counterparts. The added benefit is that the transaction will open always
in the correct database, even in a multi-db scenario.
astakos: Mimic Django transaction module

Provide a transaction module similar to Django's. The implemented
functions are commit(), rollback(), commit_on_success(),
commit_manually(), which should work exactly the same as the Django
counterparts. The added benefit is that the transaction will open always
in the correct database, even in a multi-db scenario.
cyclades: Change the imports for transactions

Do not import the transaction module from django.db. Import it instead
from synnefo.db, where the Cyclades transaction entry points are
located.
astakos: Change the imports for transactions

Do not import the transaction module from django.db. Import it instead
from astakos.im, where the Astakos transaction entry points are located.
cyclades: Add tests for transactions
astakos: Add tests for transactions
Merge branch 'feature-multi-db-transactions' of https://github.com/apyrgio/synnefo into release-0.16
docs: Update snf-image related stuff

 * Mention the fact that snf-image 0.16 is the first version that supports
   Archipelago in the upgrade notes.
 * Add instructions on how to workaround a QEMU bug found in the
   wheezy-backports package.
branding: Common cross app branding setting related to web fonts urls

Introduce BRANDING_FONTS_CSS_URLS setting to support loading of webfont
related css files from custom locations. By default fonts are loaded
from Google CDN servers. An essential utility setting for intranet
Synnefo deployments.

The `undocumented` setting UI_FONTS_BASE_URL is deprecated and no longer
used.