- Add helper isMucPM
- Move receipt build code into receipts module
- Improve receipt build code

Fixes #87

__init__.py has too many other imports, this leads to all dependencys beeing imported in the install process before they are downloaded

Fixes #85

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()