Simplify for the 80% use-case

Still needs test coverage and QA, but...  Accomplishes the following:
* Simplifies plugin initialization (single object passed to the init handler). Makes it easy-to-use, also simplifies API additions / modifications going forward.
* Converts the "enabled fields" array into an object mapping xNAL field to selector (no longer assumes the xNAL field will be a class on the specified field).
* Works in three modes now:
  * __Legacy__: The plugin can be invoked exactly as it has been previously (control rests completely outside the plugin; the plugin only takes care of form mutation).
  * __Inline config__: The plugin can be passed a JSON object structured like addressfield.json; if passed a country field in the xNAL field map, it will bind a change event to the select element and take care of things for us.
  * __Async JSON__: The plugin can be passed a JSON endpoint; it will load the JSON asynchronously and do the same change bind above.

### Full @todos
- [x] Add tests for the actual functionality of the plugin instantiator.
  - [x] Situation where options.json is a string
  - [x] Situation where options.json is a non-null object
  - [x] Situation where options.json is null (should do the same as):
  - [x] Also, update the "chain able" test to try all three scenarios above.
- [x] Update contributing documentation for installation / bower usage.
- [x] Update primary README with full details.