C++11

@springmeyer 

All expected tests are passing apart from 

```bash
Visual rendering: 2 failed / 528 passed
1) 264 different pixels:
	/tmp/mapnik-visual-images/text-halo-rasterizer-600-400-1.0-agg.png (actual)
	tests/visual_tests/images/text-halo-rasterizer-600-400-1.0-agg-reference.png (expected)
2) 282 different pixels:
	/tmp/mapnik-visual-images/text-halo-rasterizer-600-400-2.0-agg.png (actual)
	tests/visual_tests/images/text-halo-rasterizer-600-400-2.0-agg-reference.png (expected)
```

I think it's good to merge into master.