Remove tests folder and use wp-cli tool instead.

This allows more flexibility during development.

* [Blog post on benefits](http://wp.mattwie.be/2014/01/15/why-you-should-develop-plugins-with-wp-cli/#fn-431-1)
* [Docs on UnitTesting](https://github.com/wp-cli/wp-cli/wiki/Plugin-Unit-Tests)

Future commits will include woocommerce installation for testing
and {back,front}-end test cases.
Moved .gif image in own folder.
Send the content type with error responses.
Always use the stored auth_key.
Use  wp_get_referer() instead of custom.
Satisfy Travis builds (for now).
Respect user options on reordering form fields.

Form field reordering happens with JS (WP safe).
Users should explicitly allow PHP reordering (WP unsafe).

Also renamed hooked functions from custom_* to pro6pp_*.
Fix JavaScript inclusion dependencies.
Centralize the API URL.

Added an extra function to centralize the build of URL parameters.
Made the API variable a constant of the Pro6pp class.
Few minor corrections in Pro6pp class.
Updated README doc