Removed dependency on matplotlib, nbformat and pyparsing in ipython

[philippjfr]

Addresses issues mentioned in #528 partially by removing all hard dependencies on notebook and matplotlib in the ipython module. This ensures holoviews can be imported in IPython even without matplotlib and the notebook.

[jlstevens]

I suspect we'll want to add some # noqa lines here for pyflakes...

[philippjfr]

True, the nbformat lines will need it.

[jlstevens]

Looks good and does what I expect. We only need jinja2 if widgets are requested and a conditional import of archive makes sense in case pyparsing is missing.

[philippjfr]

We only need jinja2 if widgets are requested

Also used to display the notebook_extension output, but again that's only needed when working in the notebook anyway.

conditional import of archive makes sense in case pyparsing is missing.

Archive only requires nbformat. It's the parser that requires pyparsing which was already mostly working except that although the magics were wrapping the import in a try except the __init__.py was not.

[jbednar]

Does this address the issues that Ilan found when he made conda packages for the main channel, for python3? Hopefully those extra dependencies that he found would no longer be needed?

[philippjfr]

Yes it does, although we should still add notebook as a dependency to our conda package.

[jlstevens]

Yes, I was confusing the use of pyparsing for the magics with nbformat for archive. The conditional import still makes sense though...

[philippjfr]

Ready to merge once the tests pass.

[jlstevens]

Ok the tests are passing so I'll merge it now.