Reference the distributed array protocol in docs.
Correct a couple of docstrings.

One was generating a sphinx warning.
Fix API reference.
Fix another sphinx warning.
Mention optional matplotlib dependency.
Add matplotlib to travis build...

to build the api reference for plotting.py.
Example plots for protocol documentation.
Replace 'pngmath' extension with 'mathjax'...

to avoid having to install LaTeX.  I don't think we're using much if any math notation anyway.

Also removed the unused `html_static_path`.
Merge branch 'master' into feature/update-docs
Starting to standardize multiple plots.
Merge branch 'master' into protocol-example-plots
Printing engine properties.
Better docstring.
Better axis labeling.
Add a `from_dim_data` method to Context.

It actually takes a `dim_data` per process.
Adapted to generate sphinx documentation.
Fix the `Returns` part of the docstring.
Factored out properties that are the same on all engines.
Merge branch 'master' into protocol-example-plots
Merge branch 'feature/client-side-from-dimdata' into protocol-example-plots-dimdata