[WIP] Geodesic lines

This PR isn't quite ready yet. It adds support for creating geodesic lines:

![ol3-geodesic](https://f.cloud.github.com/assets/6942/2473989/d56e28c6-b046-11e3-81c0-bb9e31648197.png)

The lines are generated using an adaptive recursive subdivision algorithm that allows a tolerance (accuracy) to be specified and is independent of the map projection used. These can be use as either simple lines or as part of a generic graticule overlay.

To be done:
- [ ] The low level function (`ol.geom.flat.geodesic.line`) does not yet have a high level API.
- [ ] Wrapping over the date line currently doesn't work.