Fix id_for_label for forms with prefixes

Django supports prefixes for forms:

https://docs.djangoproject.com/en/1.4/ref/forms/api/#prefixes-for-forms

When the captcha form is prefixed, the id_for_label returned is wrong.
Now it uses the general _id variable.
Merge branch 'pr/32'

Conflicts:
	captcha/fields.py