Merge pull request #328 from plq/master

Implement Incremental XML/HTML Generation.
Merge pull request #332 from plq/master

misc bugfixes
merged protocol/_model.py back to protocol/_base.py
fix XmlAttribute serialization in dictdoc.

(XmlAttribute wrapper is simply ignored)
adapt msgpack to the new ProtocolBase api
adapt rest of the protocols to the new way of string handling in ProtocolBase api