Include mapped courses for xml4_section

Mapped courses need to be exported as part xml4_section. It’s only in
xml4_offering, that they should be omitted (because they’re mapped).

Fixes #4.
Include courses with lmsexport.include=1

Lectures and seminars are normally included in XML4 feeds. Other types
of course components that have the attribute lmsexport.include with the
value 1 should be included, too.

Fixes #5.