Added flexibility for Pronto.js. 

- Container value now can be specified and can be extended to the user's will. Key is JSON's key value, while VALUE is the HTML element to be replaced on success response.
- GET AJAX parameter name can now be changed to whatever is needed. Default's to 'pronto'.
- Response from server is passed to $.parseJSON(response) to make sure it's JSON. Otherwise, an undefined object error prompts because it's being used on the _render method.