Property "height" can now be set for a group.
Complete rework of height assignment algorithm.

Group and row heights are now considered first. Any extra space will be unused at the bottom of the card. Rows will be reduced and removed in order of priority if there is not enough space on the card.
Merge pull request #14 from finnlabs/export-card-enforced-group-heights

Property "height" can now be set for a group.