fixes issue #734 by disallowing user-defined function names as variables
Merge branch 'develop' into bugfix/issue-734-user-fun-name-var-conflict

Conflicts:
	doc/stan-reference-2.3.0.pdf
	src/test/unit/gm/parser_test.cpp
Merge pull request #759 from stan-dev/bugfix/issue-734-user-fun-name-var-conflict

Fixes #734. Bugfix/issue 734 user fun name var conflict