Add support for a new type of stanza interface: bool

The set of bool_interfaces provides default behaviour for
checking/setting the presence of empty subelements.

The prime example of this would be:

    bool_interfaces = set(['required'])

This would mean that ``stanza['required']`` would return ``True`` for:

    <stanza>
       <required />
    </stanza>

and ``False`` for:

    <stanza />

Likewise, assigning ``stanza['required'] = True`` would add an empty
``<required />`` element, and setting it to ``False`` would remove
such an element if it exists.

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