Minor fix to operator ^
got rid of commented-out lines in exponentiation_expr
missing quotation mark in Stan BNF
Merge pull request #713 from stan-dev/feature/minor-fix-for-pow-infix

Minor fix to operator ^
Merge pull request #720 from jrnold/doc-typo-in-stan-bnf

missing quotation mark in Stan BNF
adding test for generate_cpp() so it can be refactored safely