Improved example notebook generation

When examples get converted to notebooks, we use docutils to convert the ReST
paragraphs to HTML (markdown).  Docutils doesn't understand all of Sphinx's
markup, so we have to compromise in certain places.
Fix warnings generated by gallery examples
Fix integer division in CLAHE
Fix docstring markup errors
Raise an error that was suppressed to please Travis-CI
Merge pull request #1295 from stefanv/doc_build_fixes

Doc build fixes