Save donor info

First draft of saving donor info.

1. Creates a new application to store pay.gov-related functions - this division should allow us to limit which urls are accessible simply by turning applications on and off
2. Move payxml and several related functions from the view code into the new app
3. Save the generated xml when a user hits the confirmation page
4. Remove some unneeded fields from the review submission form
You bring up a good point. It's made me realize that the paygov models can't live there (lest they not be created w/ migrations). Moving the models around (and keeping only the views) is a big enough change that I want to put it in a later pull request.