adding test for generate_cpp() so it can be refactored safely
removing ability to generate main from the generator
removing 'bool include_main' argument from generate_cpp
adding test to show that 'int main' isn't in the generated code
removed ability to generate main
removing --no_main option from makefile
Merge pull request #736 from stan-dev/feature/issue-725-remove-main

Removes main from generated code