Porting IPython to a two process model using ZeroMQ

Abstract
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IPython's execution in a command-line environment will be ported to a two process model using the ZeroMQ library for inter-process communication. This will:

- prevent an interpreter crash from destroying the user session,
- allow multiple clients to interact simultaneously with a single interpreter
- allow IPython to reuse code for local execution and distributed computing (DC)
- give us a path for Python3 support, since ZeroMQ supports Python3 while Twisted (what we use today for DC) does not.