Silva.core.upgrade

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* Add upgraders to Silva 2.3b2.

* Improve the ``silvaupdate`` script with new options: ``--from-version``,
``--debug`` and ``--list``.

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* Improve upgraders to Silva 2.2.

* Add upgraders to Silva 2.3.

* Update code to be compliant with Python 2.6 and Zope 2.12.

* Add a command line tool ``silvaupdate`` to do upgrades on a non-running
Zope instance.

* Keep upgrades log in a temporary file.

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* Initial release after spliting from Silva.