New R Metrics

This adds the following three new metrics** for R:

- Relative Absolute Error (RAE)
- Relative Squared Error (RSE)
- Root Relative Squared Error (RRSE)


** Witten, Ian H, Eibe Frank, and Mark Hall (2011). Data Mining: Practical
Machine Learning Tools and Techniques. 3rd ed. Morgan Kaufman, p. 180.