Tweaks

A couple of tweaks:

- Fixed the broken default `callback` argument.
- Fixed the default value for `chosenOptions`.
- Fixed the bug where the `success` callback is called multiple times.
- Have `field` passed into the `callback` function as it's very useful.
- Simplified the JS by getting rid the usage of `if not`.

This PR contains the fix to #30 (which is covered by PR #51).