added NaN test for gamma_p
added NaN test for gamma_p
added NaN test for gamma_p
added NaN and derivative doc for gamma_p in manual
Merge branch 'develop' into feature/test-nan-gamma-p
Merge branch 'develop' into feature/test-nan-gamma-p

Conflicts:
	src/docs/stan-reference/appendices.tex
Merge pull request #975 from stan-dev/feature/test-nan-gamma-p

Feature/test nan gamma p