Deprecate prepareIDNName.

C{unicode.encode('idna')} does exactly the same as the recently
introduced L{generic.prepareIDNName}.
Do proper fallback for Request.stanzaType.

If `stanzaType` is not passed to the constructor of
`wokkel.generic.Request`, it should default to the value in the class
variable, not `get`.

Author: ralphm
Fixes: #80