Pushed to CRAN. Summary of changes since 0.4:

* Automatically recompute repulsion between labels after resizing the plot.

* Optimize C++ code for a 2.5X speed improvment.

* Add `nudge_x` and `nudge_y` to better control positioning of labels.

* Add `arrow` parameter to allow plotting arrows that point to the labeled data
  points rather than plain line segments.

* Tweak `point.padding` so that users can configure how far labels are pushed
  away from data points.

* Fix the bug when the line segment from the data point points to the origin
  at (0,0) instead of the text label.

* Change distance between segment and label in `geom_label_repel()`. Now there
  is no gap between the end of the segment and the label border.

* Fix `spring_force()` so that it never returns NaN.

* Always draw segments, even if the labeled point is very close to the label.

* Tweak forces to improve layout near borders and in crowded areas.