Merge pull request #4 from MapQuest/mla-group-symbolizer

Added test for group symbolizer, fixed segfault
Add repeat keys to collision detector for group symbolizer.
Extracted a lot of code from agg render process_group_symbolizer into common renderer.
Extract collumn collection out of process_group_symbolizer function.
The feature always needs to be copied to a subfeature in order to replace real geometry with a 'fake' point.
Merge pull request #5 from MapQuest/jmh-group-symbolizer

Separate code into common render function (etc)