added fvar signatures to unit-distr/utility.hpp
fixed partials_var typo and added more normal tests
generate_tests now generates all tests for rev and fwd mode distributions
added is_fvar_double and is_fvar_fvar_double to traits
added more specializations to utility.hpp for fvar distr
modified generate_tests to generate fvar types
split out test_fixture for distribution testing
added all working logp distr tests except finite_diffs