Using bernoulli_logit_lpmf to obtain deviance and waic

[stijn]

@llewmills

log_lik[n] = bernoulli_logit_lpmf( pulled_left[n] | a, prosoc_left[n] );

This should be

log_lik[n] = bernoulli_logit_lpmf( pulled_left[n] | a + bp * prosoc_left[n] );