Adjust for user bias

[sbyrnes]

Update the error calculation to incorporate user bias. Specifically, error should be computed as follows:

e_ij = a + b_i + b_u + q_ik * p_jk;

where

e_ij = error for the rating of j by i
a = average ratings for all items
b_i = the bias (delta against average) of item
b_u = the bias (delta against average) of user
qp = product of factored matrices