Split a swelling submitFormAsJson to smaller parts.
Filled in comments to form2json src file.
Add tests for _getFormFromOptions.
Add tests for _cloneForm.
Add a _afterInvisible method.

This method is used to insert the new JSON form into the DOM as a
sibling of the original form. This allows user to intercept its
submit action and thus it makes the submitment testable.
Add submitFormAsJsonTest tests.

Tests whether the data are actually packed and whether the variable
has the right name: Either taken from defaults, or provided manual-
ly.