Can now backtrack and change a form selection.

Form Builder saves its entire state in:
$form_state['storage']['objective_form_storage']

This now gets unset when the previous button is pressed, allowing for other
forms to be selected. Also we unset the saved values as a different form
can be presented to the user than the original so retaining the old values
doesn't make much sense.
Handle the cases where there is no step before the metadata form.
Merge pull request #94 from nigelgbanks/7.x-ingest-forms

Can now backtrack and change a form selection.