This is a quickly fix to missing plots

- Fix template
- Fix Makefile
1.  Simplifying the Makefile and adding documentation to help people find their way.
2.  Updating the README and CONTRIBUTING with more information for newcomers.
3.  Trimming whitespace from images used in the README.
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.
Bringing in changes on the master branch
More explanation of how to create new lessons
Adding image files produced from IPython Notebooks
1.  Adding pointer to IRC channel.
2.  Removing book.md during `make tidy`.
Putting '$' at the front of shell commands just like we do in lessons.
Merge pull request #428 from gvwilson/splitting-makefile

Simplifying Makefile and adding documentation.