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.
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
Merge branch 'master' into feature/update-docs
Comment out the matplotlib pip install.
denselocalarray -> localarray in docs
Merge branch 'master' into feature/update-docs
Remove distarray/local/base.py from Sphinx auto docs.