added NaN test for gamma_q
added NaN test for gamma_q
added NaN and derivative doc for gamma_q to manual
Merge branch 'develop' into feature/test-nan-gamma-q
Merge branch 'develop' into feature/test-nan-gamma-q

Conflicts:
	src/docs/stan-reference/appendices.tex
Merge pull request #974 from stan-dev/feature/test-nan-gamma-q

Feature/test nan gamma q