Made the root element parent to all subsequent elements.

The form now uses //mods:mods[1] as the root element, this allows MODS documents
that use the mods:modsCollection element as the root element to work with the
form. All references to the document context save the root element have been
changed to a parent context.

This is required to use the Audio Form with the MARCXML generated MODS.
Merge pull request #44 from nigelgbanks/7.x-form-fixes

Made the root element parent to all subsequent elements.