Add repeat-distance to text symbolizer properties.

Update collision detector to handle minimum-distance and repeat-distance seperately.
Update placement_finder to use repeat-distance.
Update group symbolizer to handle minimum-distance and repeat-distance.
Add additional test cases to demonstrate use of new attribute repeat-distance by itself and combined with minimum-distance.
Merge branch 'master' of github.com:mapnik/mapnik into jh-min-repeat-distance
Merge branch 'master' of github.com:mapnik/mapnik into jh-min-repeat-distance
Merge branch 'jh-min-repeat-distance' of git://github.com/MapQuest/mapnik into MapQuest-jh-min-repeat-distance

Conflicts:
	include/mapnik/renderer_common/process_group_symbolizer.hpp
	src/text/text_properties.cpp
Merge branch 'MapQuest-jh-min-repeat-distance'