Use Mollom for event registration

#3029

Note that a site admin should still add via ``admin/config/content/mollom/add`` the ``Events: Registration form``

I think we should not do it via installation profile, as the Mollom keys are needed.

![screenshot_6_19_13_2_35_pm-2](https://f.cloud.github.com/assets/125707/674354/d3df272a-d8d4-11e2-87c7-640463d502c8.jpg)