Creating sensible labels in FormRow automatically.

Thanks to Daniel St Jules' excellent Stringy library.
Docs and tests for Form label() and error().
Html - removing ids from labels, adding to inputs.
Using Html::label in tests.
Adding Form submit() with automatic value.
Adding Form hidden() plus improving FormTest.
FormTest cleanup.
Adding Form getFields().
Improving the intelligent labelling of FormRow.
Updating Thing::buildForm for Form rewrite.