Only included paid invoices when listing speakers with ticket
Fixes #176 -- Add abstract and description to schedule xml export
Clean up guidebook export
Allow Export of guidebook data over URLs with permission check
Fix missing import
Add schedule export support for pentabarf

This export can be used by the Engelsystem to import all talks and
keynotes for volunteer management (video, session chair, ...)

To export run:

    $ python manage.py schedule_pentabarf_export > /tmp/schedule.xcal

or for a nicely formatted xcal file:

    $ python manage.py schedule_pentabarf_export | xmllint --format - > /tmp/schedule.xcal
Merge pull request #209 from EuroPython/feature/guidebook

Update Guidebook export