Debug option

This adds a debug option and a logging wrapper to the plugin.

The benefits:
* You only get error information in the console if you want to
* The wrapper function checks if the browser supports console, so you don't get errors related to that on older browsers

The option can be accessed plugin-internally via `plugin.ajaxOptions.debug`.
The message you pass to `plugin.log()` can be any value you would use with `console.error()`. If you want to log multiple values like `console.log(a, b, c)`, just pass an array.
The second parameter tells the function whether to use `console.error()` or `console.log()`.

Hope you like the additions.