merge IPython.parallel.streamsession and IPython.zmq.session

[minrk]

Development of the Session object forked as the parallel code needed various new features. The two objects should be merged before 0.11.

This is actually a straightforward merge, and I have already done it locally, but it has to wait until the newapp code gets merged into master before a PR makes sense.

The only significant change that the parallel session object introduces is that header/parent_header/content are serialized separately, rather than as one big blob. This will affect the htmlnotebook, but so far nothing else. It's still a single multipart message with 3 JSON objects, rather than a JSON dict with 3 keys.

The newer Session is also configurable, provides HMAC message signatures, adds timestamps to headers, etc.