log1p now returns nan instead of throwing exception
removed unused includes from agrad/rev/log1m
removing unused includes from math/functions/log1p
added extra agrad/rev/log1m test
fixed math/multiply and math/divide to work with int inputs
updated TO-DO with work for next release and some longer term arithmetic stability issues
added vector-based accessors to generated model code
added enable_if to normal to restrict to agrad::var and arithmetic
test for normal to go along with new int/double/var restriction
added fwd-mode normal distribution w/o tests or vectorization
fwd-mode normal distribution impl
added new fvar, traits and model header to go along with fwd-mode normal
added identified normal mixture