Create islandora:sp-audioCModel rather than islandora:sp_audioCModel

Seems like a simple typo, but this module was creating a different content model
than it was actually using.
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 #42 from nigelgbanks/7.x-form-fixes

Made the root element parent to all subsequent elements.
Merge pull request #41 from nigelgbanks/7.x-creating-wrong-content-model

Create islandora:sp-audioCModel rather than islandora:sp_audioCModel