Tracking images produced from IPython Notebooks

[gvwilson]

1. The websites for Software Carpentry bootcamps are hosted on GitHub, which generates them automatically by running a tool called Jekyll whenever content is committed to a repository's gh-pages branch.
2. Jekyll knows how to convert compile Markdown and HTML, but doesn't understand IPython Notebooks, so if people have notebooks in their bootcamp repository, they have to run nbconvert on their own machine and add the generated .md file to the repository. (Yes, we could do something clever with post-commit hooks and continuous integration systems, but this seems simpler for our users.)
3. When nbconvert runs, it creates image files on disk for the plots and other code-generated visuals in the notebook. These image files have auto-generated names like 01-numpy_76_0.png, and the Markdown/HTML generated by nbconvert links to them.
4. We can easily add those images to the version control repository as well - but if we move cells around in the notebook, nbconvert will give them different names the next time it runs. We can add those images to version control too, but what do we do about cleaning out the old ones? One suggestion is to 'git rm' all the generated images before re-running nbconvert and trust git to detect the new image and infer that we meant to 'git mv', but that feels dangerous.

See #428 for discussion, or this issue on the IPython developers' list.

[wking]

Topics related to the larger issue of versioning generated files:

• Unwanted changes using nbconvert #430: Changes to HTML entities in IPython notebooks, possibly
  depending on installed package versions.
• Discuss: improving our build system #349: Assorted attempts and restrictions on a better IPython build
  system.
• Converting RMarkdown files to html using jekyll #92: We'd like to build HTML from RMarkdown.

[rgaiacs]

I'm closing this for now and will reopen it after we close #759.