added NaN test for falling_factorial
added NaN test for falling_factorial
added NaN and derivative doc for falling_factorial in manual
updated fwd/matrix tests
Merge branch 'develop' into feature/test-nan-falling-factorial

Conflicts:
	src/docs/stan-reference/appendices.tex
	src/docs/stan-reference/functions.tex
Merge pull request #967 from stan-dev/feature/test-nan-falling-factorial

Feature/test nan falling factorial