Add attribute text-margin in place of minimum-distance.
Handle text-margin, repeat-distance, and backward compatible minimum-distance in placement_finder. Update visual tests.
Disallow use of minimum-distance with either repeat-distance or text-margin.
Update visual test repeat-labels-1 to test minimum-distance functionality.
Minor improvement to repeat handling in collision detector.
Merge pull request #2383 from MapQuest/min-dist-compatibility

3.x -> 2.3.x compatibility for minimum-distance