Support traversable structure in JSON serializer
Support traversable structure in XML serializer
Merge branch 'master' into feature/return-traversable
Verify traversable serialization works for XP objects, too
Hand traversable XP structures through to serializer

This is done in a "streaming" fashion, it is not iterated and stuffed
into an array but instead an iterator itself is returned
QA: Add apidocs
Refactor serializers to write to a stream instead of returning a string
Initial release: REST request data wrapper

TODO: The HttpConnection class should let us upload data from streams
Optimize case for traversables in JSON serialization
Optimize case for traversables in XML serialization
QA: Fix apidocs
Flush response
Merge pull request #2 from xp-framework/feature/return-traversable

Feature: Return traversable