Add block to insert configurable form

This PR adds a new content block called ``FormBlock`` that allows pre-defined forms to be selected. The available forms are defined in the ``settings`` by adding the appropriate configuration. To make a form available, a dotted path to a Django form class is required as well as a URL to submit the form to. A example configuration would look like this:
```
FP_FORM_BLOCK_CHOICES = {
    'contact-us': {
        'name': "Contact Us Form",
        'form': 'contact_us.forms.ContactUsForm',
        'url': 'contact-us',
        'template_name': 'contact_us/contact_us_form.html',
    }
}
```

This would look something like this:

![screenshot from 2014-07-08 17 10 02](https://cloud.githubusercontent.com/assets/1236629/3506267/fa13f61e-066e-11e4-913f-dbc50c7aeaa9.png)