Rewritten directive and service to work with new reCaptcha

This allows to work with the [new reCaptcha API](https://developers.google.com/recaptcha/docs/display).

Check the differences between the new and the old implementation [here](https://github.com/VividCortex/angular-recaptcha/blob/9a402f98e4b790ad962edc12c37acc95df786b21/README.md#differences-with-the-old-recaptcha).

Closes #19