Merge pull request #5 from MapQuest/jmh-group-symbolizer

Separate code into common render function (etc)
Added another group symbolizer test.

This one shows how an obstacle under the group being displayed
will cause the whole group to not display. Above is an replica of
the same layout using non-grouped symbolizers, which shows that
only the parts which intersect the obstacle will not be drawn.
Added line placement test for group symbolizer.

This just tests placement of groups on a line with variable
spacing.
Fix bug in text placement via thunk.

The implementation shared the stored `placements_list` contents,
which meant that each loop through the renderer it was adding an
offset on top of what had gone before.

The simple fix is just to put everything back the way it was
after rendering, but this isn't ideal - it would be better to
offset only the once and keep the data the same.
Merge pull request #6 from MapQuest/mla-group-symbolizer

Added a couple of tests, fixed a bug.