Added test for group symbolizer, fixed segfault.

Segfault was due to `glyph_position` structs keeping a pointer to
`glyph_info` objects which went out of scope at a different time.
The (rather ugly) fix for the moment is to copy that information
into the thunk object.
Actually, copy constructor does OK here.
Merge pull request #4 from MapQuest/mla-group-symbolizer

Added test for group symbolizer, fixed segfault