Putting lesson URLs in configuration and taking advantage thereof.

1.  Add `lesson_repo` and `lesson_site` variables to `_config.yml`.
2.  Refer to variables in `_includes/footer.html` to link to bug reports.
3.  Modified `index.html` accordingly.
Explaining the edits needed to _config.yml
Merge pull request #92 from gvwilson/adding-website-url

Adding website url to configuration