send emails individually, so errors won't be shown to everyone
sending emails individually, tests added
Merge pull request #117 from jamox/course_notifier

Course notifier, send emails to every participant individually
Update gems.
script/export_student_events to export textual event data to json.

Snapshots are still written as ZIP files.
(use mkdir_p instead of mkdir in an export script)
now removes nil and empty strings
Merge pull request #118 from TMCee/fix-to-course-notifier

Now removes nil and empty strings
retry - now supports html formatting

uups
Merge pull request #119 from jamox/fix-to-course-notifier

Support HTML formatting in e-mails.
Update ext/tmc-sandbox.
Permit gdocs_sheet in metadata.yml to be a non-string.
Better errors if the student does System.exit().
merging