Add reCaptcha widget

Based on #77 this PR adds a captcha widget to `c2cgeoform`. To use a captcha for a form, the flag `show_captcha` can now be set when registering the schema:

    register_schema(
        'comment', model.Comment, show_confirmation=False, show_captcha=True,
        recaptcha_public_key=settings.get('recaptcha_public_key'),
        recaptcha_private_key=settings.get('recaptcha_private_key'))

Closes #77 
Closes #45 
I find it a bit inconsistent that the flag name is `show_captcha` while the options for specifying the keys start with `recaptcha_`. Is there any reason for that?