switched to no namespace version of asinh
added NaN test for asinh
added NaN and derivative doc for asinh in manual
updated fwd/matrix tests
Merge branch 'develop' into feature/test-nan-asinh

Conflicts:
	src/docs/stan-reference/appendices.tex
Merge branch 'develop' into feature/test-nan-asinh

Conflicts:
	doc/stan-reference-2.4.0.pdf
	src/docs/stan-reference/appendices.tex
Merge pull request #950 from stan-dev/feature/test-nan-asinh

Feature/test nan asinh