Build example member registration form

In order to demonstrate that Mezzanine covers the requirements in issue #14, I have
created an example member registration form.

This form demonstrates the following capabilities:
- Different field types
- Helper text
- Automatic e-mail to default recipient and to registrant (tested this by hand)
- Export of registrations in web interface or CSV format

I have implemented the form in the demo_data.json fixture. The regular way of loading it
should be sufficient here as well.

I have not resolved the following issues:
- Automatically displaying 'required' as helper text with required fields
- Implementing an Integer field type.

I feel those are not that important to fix - they are not explicit requirements in the
issue, anyway.

I have not yet implemented a CAPTCHA.
Add integer field type

I did not like the fact that there was no integer field type. Also, I saw that you could add custom field types for forms, so I did.

In issue #14, this helps to fulfill the need for a house number field that checks whether a number is indeed entered.

I also added demo data to demonstrate the functionality. I have manually verified that it also sends out correct values (i.e. without decimal) in the e-mail confirmation.
Add CAPTCHA field type to forms app

* Added the CAPTCHA field type to the forms app, using the same method as used in the
previous commit for the Integer field type.

* Added demo data to the fixture to demonstrate this functionality. The form on
jd.local:8000 contains a CAPTCHA field.

* Tested by hand that functionality works in combination with existing form functionality.
Merge branch 'feature/registrationform/#14'