softmax unfolded rev mode, added log_softmax and unfolded rev mode
added log_softmax rev mode
all tests passing for softmax functions and categorical distributions
finished merge vs. new Boost, fixing matrix includes
removed categorical_log, added vector/matrix log_sum_exp, improved categorical_logit speed when not vectorized
merged with develop, resolved conflict in function_signatures.h
Merge pull request #166 from stan-dev/feature/softmax-unfold

Feature/softmax unfold
Merge branch 'develop' of https://github.com/stan-dev/stan into feature/increment_log_prob