* (2011-09-12) Fix 'cluster' option in coxreg (coxph).

* (2011-08-29) 'centering' and 'newdata' must be performed in a sensible way, 
e.g., 
1. Refering to the original variables and not the 'design matrix'.
2. Factors should not be "centered" (keep a "reference group").
+++++++++++++++++++++
