added NaN test for inv_logit
added NaN test for inv_logit
added NaN and derivative doc for inv_logit to manual
merge develop
Merge branch 'develop' into feature/test-nan-inv-logit

Conflicts:
	src/docs/stan-reference/appendices.tex
Merge pull request #990 from stan-dev/feature/test-nan-inv-logit

Feature/test nan inv logit