Add @api stable annotations to ol.proj

This PR adds `@api stable` annotations for projection-related objects.

Things to note:

- The `getWorldExtent` and `setWorldExtent` methods and the `ol.ProjectionOptions#worldExtent` option are marked as "experimental". These are related to `ol.Graticule`, which I do envision marking as stable for v3.0.0.

- I initially intended to mark `ol.proj.Projection`, `ol.proj.addProjection`, and `ol.proj.addCoordinateTransforms` as "experimental", but I realized that they are used when working with custom projections (as in the [wms-custom-proj](https://github.com/openlayers/ol3/blob/master/examples/wms-custom-proj.js) example). So I've decided to make them "stable".

Please review.