Browsing French translation

1699.

To use this, simply have your class inherit from <code class="API" base="twisted.persisted.styles">Versioned</code>. You don't have to do this from the very beginning of time: all objects have an implicit version number of <q>0</q> when they don't inherit from Versioned. So when you first make an incompatible data-format change to your class, add Versioned to the inheritance list, and add an <code>upgradeToVersion1</code> method.