Fixed #1 -- Add support for template choices

Having to input the template path to the form / success template is a
pain. This commit adds the ability to define the settings variables
`DYNAMIC_FORMS_FORM_TEMPLATES` and `DYNAMIC_FORMS_SUCCESS_TEMPLATES` and
offers their values as choices.
Fix some doc links
Allow delayed action and dynamic_field registration

Use an iterator instead of directly returning a list of 2-tuples. This
allows registration of actions and dynamic fields after the `FormModel`
/ `FormFieldModel` have been loaded
Allow dynamic form fields to exclude their value from being processed by the form actions
Dropped Python 3.2 support
Fixed #2 -- Add support for django-simple-captcha
Set some default ADMINS in the example project to make the mail action actually do something.