Added support for stoken

[gbarrett]

This adds support for stoken as described on this page:

https://developers.google.com/recaptcha/docs/secure_token

I did not include anything in the noCaptchaProvider section for setting default options. This token includes a timestamp, so it seems more appropriate to set it when creating the recaptcha instance as opposed to in a global configuration.

[vepasto]

Thanks @gbarrett!

Could you add documentation to readme, please?

[gbarrett]

Certainly, I've added a line in the parameters section. I think that it, combined with the link, provides enough information. Let me know if you think anything else needs to be added.