Fix bug in pair layout with a single column.

Code had assumed that (0, 0) would be the center of the layout
and represent "no offset", but this isn't the case.
Added test for repeat key in group symbolizer.

Test shows two items with different repeat keys ("foo" and "bar")
and rows of other group symbolizers sensitive to either one. The
reference images show that the "bar"-sensitive rows are displaced
by the "bar" item, but not the "foo" item.
Don't center the bbox, just apply origin offset.

As @jhollinger2 says; "This layout is not supposed to center the
boxes on the y-axis", instead, all that is necessary is that the
offset back to the (geometry) origin is applied.
Merge pull request #10 from MapQuest/mla-group-symbolizer-rpt-test

Added repeat-key test.