Fixes #84

Before we raised CommonError even if the stanza was not an error node.
Now we raise the dedicated StanzaMalformedError.

This is used for example to advertise a new MUC after destruction

<gone>xmpp:new@chat.example.org?join</gone>

- Validate JID on incoming voice requests
- Add more data to VoiceRequest
- Add approve_voice_request()

- Make get_field_value public
- Add has_field()

Make stanza object available in DiscoInfo and derive some propertys from it

- Parse application errors
- Parse text elements in all languages

Get the xml:lang attr, if not found traverse all parents

Old ejabberd servers dont accept missing value elements in text fields.
This now does not delete the value element anymore if it's empty.

Attribute contains a bare JID object if the Message or Presence is from a MUC

- Validate while parsing into JID
- Add dedicated methods for the different JID parts
- Follow RTC7622 exactly
- Add unit tests