Autobuild lessons

The highlights:
 * A few minor changes to get lesson_format running on heroku (e.g. git ops are now over https; we use local binaries for zipping and pandoc-ing if they’re present.) Everything *should* still work locally, but do test that.
 * build.py is refactored so it can be called more easily from an external script.
 * autobuild.py has been added, which calls build.py but also does various git and github operations. This is what @codeclubrobot runs.
 * if a `pdf_generator` is specified when calling build.py, all lesson PDFs will be auto-generated. The ones referenced in the manifests will be ignored.

Fixes #45.