Interface to versioned dictionary
<p>I have an versioned document store which I want to access through an dict like interface.
Common usage is to access the latest revision (get, set, del), but one should be able to access specific revisions too (keys are always str/unicode or int).</p>

<pre><code>from UserDict import DictMixin
class VDict(DictMixin):   
    def __getitem__(self, key):
        if isinstance(key, tuple):
            docid, rev = key
        else:
            docid = key
            rev = None  # set to tip rev

        print docid, rev
        # return ...

In [1]: d = VDict()

In [2]: d[2]
2 None

In [3]: d[2, 1]
2 1
</code></pre>

<p>This solution is a little bit tricky and I'm not sure if it is a clean, understandable interface. Should I provide a function </p>

<pre><code>def getrev(self, docid, rev):
   ...
</code></pre>

<p>instead?</p>
