On-the-fly static rendering of IPython notebooks

If enabled (via the ipython_notebook.render configuration setting),
visiting the blob view on any .ipynb file shows a 'View' button in the
action bar. Clicking it triggers a call to IPython nbconvert and the
result is shown verbatim. IPython (>= 1.0) must be installed on the
server.

Note that this is a very ad-hoc patch. For example, rendered notebooks
are not cached meaning that performance may be suboptimal.
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