Don't rotate horizontal labels

[kkaefer]

Right now we rotate horizontal labels when the user rotates the map. This is because we'd need to compute a collision map as suggested in http://arxiv.org/pdf/1104.5634v1.pdf and integrate that with the current label show/hide code.

[ansis]

I just merged support for this into gl. It still needs some cleaning, more tests and performance improvements, but it works! Overall, I'm happy with the quality. It could be improved by adding a second range for a higher zoom level, but even without it its pretty good.

[kkaefer]

Yay! This looks very good! Nice work!

[ansis]

Done. Remaining problems have separate issues