API - Basic API calls

@sdaityari you need to do some research and check if current ATutor version supports anonymous functions or passing a function as an argument. If yes then it is great and we can have another option passed into `api_backbone` $callback which would be a function to execute custom code with argument $result returned from queryDB. If current ATutor version does not support this then we need to think of the alternative ways. I'm worried about an example for delete API here where you need to execute extra logic. But also worried about POST APIs since they have to return id of the just created new entries