always use Session.send

[minrk]

This commit removes all explicit calls to socket.send/socket.recv, and replaces them with session.send/recv. This allows for later expansion of the send/recv protocols, and eases eventual migration to StreamSession object developed in newparallel.

Some adjustments were made to the Session to accomodate this:

• Session.send can take a Message or dict to allow for multiple sending of the same message
• Session.send returns a dict instead of wrapping it in a Message
• Session.recv always returns a tuple of length two: (ident,msg), where ident is None if there was no identity prefix, and msg is None if EAGAIN was raised (indicating no message).

Everything else should function the same.

[ellisonbg]

session.py

• In send, rename msg_type to msg_or_type to emphasize it can be either.
• Add docstrings to the send, recv and msg methods, that clarify what the
  methods take and return.
• Remove the comment on L98.
• Are you using the Message class in parallel? Should be get away from it?
  What do your recommend? I like that recv returns a raw dict. I think
  session should deal primarily with dicts at its core.

displayhook.py

• Where was the parent being set before near L17? Was this a bug?

Other than that, this looks good. Why don't you fix these things and go
ahead and merge.

[minrk]

session.py

• docstrings updated, comment removed.
• No, I approximately never use the Message class in parallel. I use it once or twice in the Engine (not kernel) and client for attr-access, but nowhere internal.

displayhook.py

The parent setting isn't different at all. Note that L18 is unchanged, and L17 doesn't close its parentheses.