Fixes ipython printing extension

This fixes two things.

First, the extension had to be reloaded to import the extension. Fixes issue #3619 [1].

Second, this has been broken since IPython moved IPython.zmq to IPython.kernel.zmq. This PR fixes this. Note, there are some git versions of IPython where this will still break (between moving the zmq module and changing the version to 1.0.dev), but it should work for all stable releases and will work for the current git version. The versions that won't work are between 1/29 [2] and 2/15 [3]. This was brought up in #1757.

[1] https://code.google.com/p/sympy/issues/detail?id=3619
[2] ipython/ipython@37f32253e06e216b9690c52dd6f554992d5b82b6
[3] ipython/ipython#2927