DOC: enable parallel building of the docs
Sphinx supports building the docs in parallel using multiple processes (with eg `-j 4`), see http://www.sphinx-doc.org/en/stable/invocation.html#cmdoption-sphinx-build-j 

I quickly tested this, and that seems to reduce on my laptop (with 4 processes) build time from ca 17 min to 8 min. 
This also takes a lot more memory, so not sure if we want to enable it by default, but it would be nice to at least give the possibility to specify this with `python make.py html`.

@jreback do you know if we can use multiple processes in parallel on travis?

What this needs:

- update the sphinxext (ipython directive and numpydoc) to allow parallel read (incorporate some upstream changes into our vendored versions)
- update `make.py` to be able to pass the `-j` argument

