Blender Git Loki

Expose Bullet rotational spring settings in the UI.

Bullet spring constraint already supports rotational springs, but
they are not exposed in blender UI, likely due to a simple oversight.
Supporting them is as simple as adding a few DNA/RNA properties
with appropriate UI and passing them on to Bullet.

Reviewers: sergof

Reviewed By: sergof

Differential Revision: https://developer.blender.org/D2331