Make extending stanza objects nicer.

A stanza object may add is_extension = True to its class definition
to provide a single new interface to a parent stanza.

For example:

import sleekxmpp
from sleekxmpp import Iq
from sleekxmpp.xmlstream import ElementBase, register_stanza_plugin, ET

class Foo(ElementBase):
    """
    Test adding just an attribute to a parent stanza.

    Adding subelements works as expected.
    """
    is_extension = True
    interfaces = set(('foo',))
    plugin_attrib = 'foo'

    def setup(self, xml):
        # Don't include an XML element in the parent stanza
        # since we're adding just an attribute.
        # If adding a regular subelement, no need to do this.
        self.xml = ET.Element('')

    def set_foo(self, val):
        self.parent()._set_attr('foo', val)

    def get_foo(self):
        return self.parent()._get_attr('foo')

    def del_foo(self):
        self.parent()._del_attr('foo')

register_stanza_plugin(Iq, Foo)

i1 = Iq()
i2 = Iq(xml=ET.fromstring("<iq xmlns='jabber:client' foo='bar' />"))

>>> i1['foo'] = '3'
>>> i1
'3'
>>> i1
'<iq id="0" foo="3" />'
>>> i2
'<iq id="0" foo="bar" />'
>>> i2['foo']
'bar'
>>> del i2['foo']
>>> i2
'<iq id="0" />'

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