Fix broken links in documentation

[jlstevens]

This PR addresses issues #198 and #1251 regarding broken links.

[jlstevens]

I'll run a website build and then run a link checker on dev.holoviews.org.

[philippjfr]

Would be good if you could update this paragraph in the index.rst with the most recent versions:

HoloViews works with Python 2.7, 3.3, and 3.4 <https://travis-ci.org/ioam/holoviews>_.

HoloViews is pure Python, but it also provides optional extensions
enabled with hv.notebook_extension() above that make it integrate
well with Jupyter/IPython Notebook <http://ipython.org/notebook>_ 2
and 3.

[jlstevens]

I rebased to get doc building working again. Making the suggested change now and then rebuilding the website.

[jlstevens]

Tests are now green and build.holoviews.org has been rebuilt and all the links checked. I think this is ready to merge now.

[philippjfr]

Looks good.

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