Initial code import.
Standardize default template names
Improve ListBasedMixin and SubscribeFormView

* Make it easier for ListBasedMixin to get a list_id from
  a source other than the url kwarg
* Pass the merg_vars from the form into the form_valid success
  return
Improvements to form generation

* Add ability to turn double_optin off
* Added in some fields and field placeholders
Fix USPhoneNumberField adding
Add date field
Implement URL field
Implement radio widget