Refactored for extensibility

When there are docs, I'll add this to them, but:

* `populate.forms` should have a module for each available form.

* Each form name (module name within `populate.forms`) should correspond
  to a PDF of a blank form within `populate/blank_forms`.

* Each form's module should contain a function `fill_out_form` that
  accepts the following positional arguments:

  * The blank form's path.
  * The settings dict with the field boundaries.
  * The contents dict to use when populating the form.