sample 'newk' directly from the log-probabilities

1. sample 'newk' directly from the log-probabilities
2. performance enhancement:
a. more succinct log output for 'newk' in Z
b. output elapsed time ('etimes') in 'prfmstats'
c. avoid truncation in iteration curves ('prfmstats')
d. save 'K' in 'init.mat'
3. various bug fixes
a. Z>0.5 -> Z>0 ('logpibp')
b. calculate 'fobj' only for existing features ('train_ipm3f')
c. add convergence control for EM as well ('train_ipm3f')