Contribution: modified export_html

Hi

I've added an option 'room' to the output module export_html. This is
analogous to 'class' - it outputs a timetable for each room showing which the
subject, teacher and class for each lesson. I'll need this for rooms such as
the computer room which are booked by other teachers on an ad hoc basis.

Reply if you'd rather have a diff, patch, etc.

KR

Nick