A little tidying of the node comments.
Remove modules code in favour of attributes

Models still not added back, but everything else is approximately there now.
Still some things missing from subtrees, and I might use active binding
functions to simplify working with some pseudo-fields, but the basic
idea is there and all tests pass.

This breaks compatibility with previous versions, but we *are* pre-alpha.

Closes #25
Tidied up code generation considerably
Move tests to new testthat format