This is a quickly fix to missing plots

- Fix template
- Fix Makefile
Merge branch 'fix-402' of github.com:r-gaia-cs/bc into fixing-missing-plots
1.  Merging fixes for missing image files from @r-gaia-cs.
2.  Putting IPython Notebook stuff in a separate Makefile so it won't inadvertently trigger when doing a regular site build.
    This ensures that people who *don't* have IPython installed,
    and don't want to re-create the Markdown files for notebooks,
    won't accidentally try to do it.
Updating documentation on build process
Updating contribution guidelines based on feedback from @r-gaia-cs
New Makefile holding IPython Notebook conversion rules
1.  Refactoring code to allow for optional fields.
2.  Making `enddate` optional.
3.  Adding optional `eventbrite` field check.
4.  Adding always-pass checks for `venue` and `address` fields for completeness.
5.  Adding tests for the above.