Can now uninstall islandora and dependant solution packs.

Drupal requires modules to be disabled before uninstalling. Thus classes Drupal
would otherwise autoload when needed are no longer present in the registry
during the uninstall hook. Also the required hook for uninstalling
__islandora_required_objects__ is not availible.

Now required classes for uninstalling are manually loaded, and the module file
which contains the __islandora_required_objects__ hook is manually loaded as
well.
Merge pull request #430 from nigelgbanks/7.x-uninstall-issues

Can now uninstall islandora and dependant solution packs.