VCardImage parsing failures

[Jajcus]

[source:trunk/pyxmpp/jabber/vcard.py##L394 line 394 of vcard.py] fails when there is no key {{{"value"}}} in the {{{rfc2425parameters}}} dict object. The current code below:
{{{
if rfc2425parameters.get("value").lower()=="uri":
}}}
when {{{"value"}}} does not exist in the dictionary, {{{get("value")}}} returns {{{None}}}, resulting in {{{None.lower()}}} which finally throws an exception:
{{{
AttributeError: 'NoneType' object has no attribute 'lower'
}}}
This is caused by the default return value from get, which itself defaults to {{{None}}}. My fix is to include a default value of empty string:

{{{
if rfc2425parameters.get("value", '').lower()=="uri":

}}}

I also had some trouble with not all of the attributes being initialized when parsing from !rfc2425 versions, my fix is to move the initialization with {{{None}}} out of the if statement on [source:trunk/pyxmpp/jabber/vcard.py##L370 line 370 of vcard.py].
{{{
if isinstance(value,libxml2.xmlNode):
self.uri,self.type,self.image=[None]_3
}}}
becomes
{{{
self.uri,self.type,self.image=[None]_3
if isinstance(value,libxml2.xmlNode):
}}}

Copy of the PyXMPP issue Jajcus/pyxmpp#13